AI'TH(IR'S ABI(TMACT OF THIS P A P E R ISSUED RT THE: BIBI,IOGP.APHIC SERVICE. OCTOBER 8

THE ARTERIAL SYSTEM O F A TWO-HEADED PIG EMBRYO

MABEL BISHOP

Anatomical Laboratory of the Universi ty of Chicago

THREE FIGURES

This last of a series of three,studies of terata occurring free in nature has had a two-fold incentive-to compare the behavior of the arterial with the nervous system already de- scribed in the embryo ('21) and to keep faith with a promise made in a still earlier report on tlie arterial systems in a series of terata ( '08, p. 446).

The teras under consideration is a dicephalous, bilaterally symmetrical pig embryo 22 mm. in length. It has been fully described and figured in the 1921 paper ' (pp. 381-384) ; there- fore, with reference to the present study, the reader's atten- tion is called only to tlie fact that the head of the embryo is incompletely doubled, consequently i ts two members a re designated head A (or component A) the right member, head B (or component B) the left member. The term teratological head is used to include both members and refers to the head of tlie embryo treated as a unit. 'Inner side' and 'outer side' refer to the two sides of each head component.

As in earlier studies of terata, wax-plate reconstruction models have been made at a magnification of 25 diameters, using 1-mm. wax plates to represent every other section. Both model and microscopic sections have been carefully checked by constant reference to normal embryos of 20, 22, and 25 mm. in length. The drawings for the model of the heart and arteries were made with a Spencer Delineoscope at the Carnegie Institute of Embryology, but the model was

205

2006 MABEL BISHOP

made in tlie Department of Anatomy, University of Chicago, and the study completed here. Both drawings and iiioclcl were made by the author.

THE HEART

The lieart presents no unusual appearance either in tlie gross as seen in the model or in the microscopical transections of the embryo.

THE ARTERIES

The pulmonary artery and the systeinic aorta arise from the heart in normal manner and behave normally in tlieir fur- ther course through the portion of the embryo studied.

Ayter ies arisiizg f r o m the aorta r i 1 he coronary arteries arise ii? normal manner ant1 pursiue

a normal course. Frorri the convexity of the aortic arch a short inrioniiriate

artery arises, from tlie termination of which the left coiniiioii carotid, the right corniiion carotid, and the right subclavian take origin as in normal embryos. The left subclavian arises independently from the posterior extremity of the arch. Each subclavian in turn gives off its usual branches in normal manner and may be rasily traced into tlie developing limb bud, but since our interest lies mainly in arteries further forwartl, a detailed description of the peripheral distribution of these several branches is not given except for the vertebral arteries (p. 213). However, the principal branches arc labelwl in figure 1. The costocervical arteries arise in commoii with tho vc r t c b r a1 s .

Following the right and left common carotids forward througli the neck, one observes 110 marked deviation froin nor- mal prior to the division of each into exteriial and intcriial carotids. From this lcvel the arterial supply to cacli lieacl component must bc considcrril separately for eavli outer aiitl inner (median) moiety.

-4RTEnIAL SYSTEM O F A TWO-HEADED PIG EMBRYO 207

Arterial supply to head B

The left common carotid furnishes the arterial supply to component B. It gives rise to small iiiferior thyroid and esophageal rami, and a larger thyrolaryngeal artery, each having normal distribution. At the upper level of the thyroid gland i t gives off a stout branch which turns toward the median plane of the embryo, and just rostral to this another artery which turns lateralward and is distributed to the left (outer or normal) half of head B. This more rostral artery is therefore the left external carotid artery of head B (fig. 1). Near its origin i t sends a lingual branch into the left half of head B, and a short external maxillary artery to the outer side of the head, from which rami are readily traced into the left submaxillary gland. A detailed description of the ar- teries of the normal side of the head component is not in- cluded, but is referred to in a general way in considering the arteries of the median region (p. 215).

The branch from the common carotid just mentioned that turns toward the median plane courses rostralward along the external lateral surface of Reichert 's cartilage, and upon reaching the midplane of the embryo continues forward in the midline for the remainder of its course. It gives off a well- developed lingual branch to the right half of the tongue of component B, and is interpreted as the right external carotid of this head member. At the root of the tongue a short anas- tomotic branch connects it with the left lingual (figs. 1 and 2). It then runs along the midplane of the embryo and supplies the left half of the tongue of head A and the right half of the tongue of head B. This portion of the artery I have designated a median external carotid (probably conjoined). I ts further course is considered in the section Arteries of the median region (p. 215).

The origin of the right and left external carotids of head B from a single common carotid, viz., the left common carotid of the teras, may be looked upon as the first important regu- THE ANATOMICAI, RECORD, TOT,. 26. NO. 3

“08 MABEL BISHOP

latory adjustment of the anterior arteries (except the median vertebrals, p. 214), since there is a right and a left common carotid for the embryo as a whole, but not for each head member.

The occipital arteries do not arise in relation to the es- ternal maxillary and lingual branches, as normally expected, nor with the internal carotid by a common stem, as sometimes occurs.

Rostra1 to the origin of the right and left external carotids (head B), the common carotid continues forward as a left in- ternal carotid of tlie teratological head (fig. 1). I t gives rise to the left stapedial artery of component B and continues on its way to the posterior end of the hypophysis of this head member. Here it bifurcates, thus giving rise to a right and left internal carotid for head B, the left artery passing around the outer surface of the hypophysis and diencephalon, the

They seem not to be present.

ABBREVIATIONS A , component A (head A) of the ter-

atological head Aa.znfr., arteriae infraorbitales A.ao., arcus aortae Aa.u.m., arteriae vertebrales mediae

A.i?l.fr., arteria, infraorbitalis AZ.i.Zt.A., arteria alveolaris inferior

sinistra (head A) AZ.i.rt.B., arteria alveolaris inferior

dextra (head B) An., arteria anonyma Atr.Zt., atrium sinistrum Atr.rf . , atrium dextrnm B., component B (head B) of the ter-

Br.Et., arteria braehialis sinistra Br.rt., arteria brachialis dextra Bs.A., arteria basilaris (head A ) Bs.B., arteria basilaris (head B) C,, costa prima C.a.Zt., arteria eerebri anterior sinistra f .a .r t . , arteria eerebri anterior dextrs

(separate)

a tological liead

Cb.p.Zt., arteria cerebelli poster. sin-

Cb.p.rt., arteria cerebelli poster. dextra C.c.Zf., arteria carotis commuiiis sinistra C.c.rt., arteria carotis communis dextra C.e.Zt.B., arteria earotis externa sinistra

(head B) C.e.m., arteria carotis externa media

(Con joined ? ) C.e.rt.A., arteria carotis externa dextra.

(head A ) C.e.rt.B., arteria carotis exteriia dextra

(head B) Cg.e.a.m.co., cartilages of the external

auditory meatuses (conjoined) Cg.Ws.A., cartilago nasi (head A ) Cg.Ns.B., cartilago nasi (head B) C.i.e.co., capsules of the internal ears

(conjoined), including stapes C.z.Zt., arteria carotis interna sinistra. C.i.rt., arteria carotis interna dextra CZ.ns., arteria ciliaris longa nasalis G.Zt., arteria coronaria (coriiis) sinistra

istra

ARTERIAL SYSTEM OF A TWO-HEADED P I G EMBRYO 209

Cl . t . , arteria ciliaris longa temporalis C.m.Zt., arteria cerebri media sinistra C.m.rt., arteria cerebri media dextra C.rt., arteria coronaria (cordis) dextra d.a., Ductus arteriosus d.slg., ductus glandulae sublingulae d.smx., ductus glandulae submaxillariae E.n.me.co., external auditory meatuses

E.a.me.Zt.B., left external auditory

E.a.me.rt.A., right externaI auditory

E.Zt.A., left eye (head A) E.Zt.B., left eye (head B) E.rt.A., right eye (head A) E.rt.B., right eye (head B) g.Gn.co., ganglion geniculi (conjoined) gZ.smr., glaiidula submaxillaria g.SpZ.Zt.A., ganglion sphenopalatinum

g.SpZ.rt.B., ganglion sphenopalatinum

Hyp., hypophysis LA., arteria lingualis sinistra LA., arteria Iingualis dextra M.e.Zt., arteria maxikr is externa sin-

M.e.m., arteria maxillaris externa me-

M.e.rt., arteria maxillaris externa

MMk.cg.co., cartilago Meckeli (conjoined) Mlc.cg.Zt., cartilago Meckeli sinister Mk.cg.rt., cartilago Meckeli dexter mm., muscle mass m.Ms.Zt.A., musculus masseter sinister

m.Ms.rt.B., musculus masseter dexter

.nz.Pt.Zt.A., musculus pterygoideus sin-

ni.Pt.rt.B., musculus pterygoideus dex-

N.infr.lt.A., nervus infraorbitalis sin-

(eonj oined)

meatus (head B)

meatus (head B)

sinister (head A)

dextrum (head B)

istra

dia (conjoined)

dextra

(head A )

(head B)

ister (head A )

ter (head B)

ister (head A)

N.infr.rt.B., nerviis infraorbitalis dex-

N.p.s.m., nerws petrosus superficialis

nt., notochord Op.Zt., arteria ophthalmica sinistra Oprt . , arteria ophthalmica dextra P., plexus P.bs., plexus basilaris Pul., arteria pulmonalis r.c., ramus circumflexus rd., arteria radialis r.d.a., ramus descendens anterior r.i., ramus inferior r.s., ranius superior S.Zt., arteria subclavia sinistra S.rt., arteria subclavia dextra Xp.v.Zt., arteria spinalis ventralis sin-

Sp.v.rt., arteria spinalis ventralis dex-

Xtp.Zt., arteria stapedia sinistra (head

S t p a , arteria stapedia media (con-

Stpxt. , arteria stapedia dextra (head

T.A., tongue of head A T.B., tongue of head B T.e.Zt., arteria thoracica externa sin-

T.i.rt., arteria thoracica interna dextra T.e.rt., arteria thoracica externa dextra T.i.Zt., arteria thoracica interna siii-

istra Tm.smxZ.gl.co., terminals of submaxil-

lary and sublingual glands (con- joined)

ter (head B)

major

istra (head B)

tra (head A)

B)

joined)

A )

istra

Tr.c.c.e., truncus eostocervicalis Tr.oc.Zt., truneus omocerviealis sinister l'r.oc.rt., truncus omocerviealis dexter ul., arteria ulnaris V.Zt., arteria vertebralis sinistra V.rt., arteria vertebralis dextra F'tr.Zt., ventriculus sinister F'tr.rt., ventriculus dexter

Fig. 1 A free-hand drawing from the model of the heart and large arterifs in ventral aspect. Owing to the cranial flexure of the teratological head, it has been necessary to represent tlie arteries a~ though the cliiiis of the head coni- ponents had been tilted high and pushed backward, bringing the arteries at dif- ferent levels into an artificial sequence but retaining their topographic relations. I t has been necessary, also, to represent the head meinbeis spread apar t in order to give space for the arteries of the median region; this has exaggerated the length of some and the distances between others. Rostrally directed arteries a re towird the top of the page, caudally directed ones toward the bottom. The right-hand component (A) therefore appears at the render’s left, the left component (B) at tlie reader’s right.

The position of plexuses in all the figures is indicated by open circles :bnd the letter P. Abbreviations of the word ‘artery’ is omitted from the labels.

Arteries a re represented in solid black.

210

ARTERIAL S Y S T E M OF A TWO-HEADED P I G EMBRYO 211

right one around the inner (median) surface (fig. 1). At the base of the right and left optic nerves, respectively, the def- initive anterior and middle cerebral arteries take origin. From the root of each anterior cerebral a well-defined oph- thalmic artery accompanies its optic nerve giving off both short and long ciliary arteries, enters the optic nerve, and as the central artery of the retina breaks up into the hpaloid plexus within the optic cup as in normal embryos (fig. 1).

Fig . 2 Free-hand composite drawing from the sections to show schemat- ically the topographical relations of the inferior alveolar arteries and the arterial complex around the median exteriixl auditory meatuses to the median exteriial carotid artery.

There appears to be no closure of the circle of Willis by either anterior or posterior branches from the internal carotid, hence no connection with the basilar arteries, to be described presently.

MABEL BISHOP

D I

ARTERIAL SYSTEM O F A TWO-HEADED P I G EMBRYO 213

Arterial supply to head A The distribution of arteries in head A is the same as in

head B, with the exception of the left stapedial artery. This arises just behind the hypophysis, passes to the midplane of the teras and through the conjoined cartilages of the internal ear capsules and stapes (figs. 1 and 3 ; '21, figs. 14 and 15). It thus becomes a median stapedial artery, probably conjoined, and compensates the absence of a right stapedial artery for liead B. Its further course followed in the section on Arteries of the median region (p. 215 ).

T H E VERTEBRAL ARTERIES

a. Of the outer sides

The vertebral arteries of the outer sides of tlie teratological head behave alike and may be described together. They arise from the subclavians as expected normally, but it will be shown presently that they are in reality the right vertebral of component A and the left of component B. Each passes ros- tralward along tlie lateral sides of the bodies of the cervical vertebrae, and through tlie iiitervertebral foramen between the fifth and sixth verterbrae sends inward to the ventral sur- face of the spinal cord a stout branch which is the ventral spinal artery for its respective head moiety (fig. 1). They pursue a parallel course, one on each side of tlie ventral neu- ral ridge, which runs along the midventral surface of tlie spinal cord ('21, p. 388), and on the ventral surface of the medulla oblongata anastomose to form a conjoined basilar plexus extending as far forward as the level of the superficial origin of tlie seventh and eighth cranial nerve roots. This plexus is reinforced by another branch from the vertebrals a little farther forward. From the left half of the forward end of this plexus an artery emerges which soon ends blindly in mesenchyme, but from the right half a stout basilar artery emerges which bifurcates into a Y-shaped artery. The right limb of the Y passes to head A, where it ends abruptly to the

214 MABEL BISHOP

outer side of the hypophysis; at least it cannot be traced farther in tlie sections even with high-power lens. At the level of the sixth nerve it gives off a posterior cerebellar artery. The left limb of the Y-shaped basilar turns toward liead B and behaves exactly in the manner described for head A. Neither is in communication with the internal carotids

We have here an unusual coridition f o r pig. To recapitu- late, there is found a Y-shaped artery along the ventral SIT-

face of the conjoined medulla oblongata in the position of tlie basilar artery of a normal adult, and the artery under consid- eration has therefore been designated a ‘Y-shaped basilar. ’ It should be noted that normally the blood flow in the basilar artery of a pig is spinalwarcl, derived chiefly from the in- ternal carotids by way of their posterior rami. But these rami are absent in tlie teras, and the vessel here called ‘basi- lar’ derives its blood from the ventral spinals of the two com- ponents-an interesting regulatory adjustment. No connec- tion between the so-called basilar and the internal carotids exists in this embryo, though a participation in the rete mira- bile of the hypophyscal region cannot be excluded.

(fig. 1).

b. Of thc i .~ii ier sides.

In tlie description of the spinal nerves in the 19’31 paper, four conjoined median spinal nerves were demonstrated dor- sal to the esophagus ( ’21, p. 388; figs. 5 and 20). The third nerve is the longest, extending down into the mediaiiastinum as a well-developed nerve trunk, and even farther than tlie sections of the embryo in my possession. A pair of diminu- tive, but separate vertebral arteries accompany tliis nerve through the neck to a level about opposite the upper limit of the body of the thyroid gland (fig. 1). These median vertc- bra1 arteries, representing diminutive left vertebral of head A and right vertebral of head B, take a slightly minding course apparently, f o r in some sections they lie lateral to the nerve, in others on its dorsal surface. This is of minor im-

ARTERIAL SYSTEM O F -4 TWO-HEADED PIG EMBRYO 215

portance, and may indeed be an artifact due to mechanical pressure during preparation of the microscopic sections. In an occasional section the arteries seem to be confluent, but this is due, I think, to fine branches which meet sometimes on the dorsal surface, sometimes on the ventral, and represent perhaps diminutive dorsal and ventral spinals.

The origins of these median vertebrals are not seen in the sections, but the inference is that they arose normally from the right and left subclavians, or perhaps from the lower ends of the vertebrals described for the outer sides, and subse- quently lost connection with parent stem. It is obvious also that they do not enter into the formation of the basilar plexus, wliich is made up of the ventral spinal artery from the right vertebral of head A united to the ventral spinal from the left vertebral of head B, and therefore, by definition, conjoined

T'ertebrals in relation to the other median spinal nerves are not apparent. All of the vertebrals seem to have departed considerably from normal expectations.

(fig. 1).

ARTERIES O F THE MEDIAN REGION

The structures within thc left half of component A and the right half of B lie within the median region, i.e., the area be- tween the diverged snouts. Here the arteries manifest nor- mal, or near normal, morphological relations to the organs supplied by them, but the restricted and anatomically dis- turbed positions of some of the organs themselves complicate the topographical picture, as may be expected when it is re- called that the two heads are conjoined in the rhombenceph- alic region of the brain ('21, summary 10 and 11) and struc- tures at that level have been the most disturbed in position. Also, the median plane of the teratological head represents juxtaposed external lateral surfaces of heads A and B (rough- ly speaking, the cheeks) in consequence of which the most superficial structures of the two head members lie nearest the midplane of the embryo, or have been everted in the direction of least resistance.

216 MABEL BISHOP

This median region was therefore carefully studied and certain anatomical landmarks identified and summarized in the study of the nervous system (’21, p. 413); so only the arteries are given special attention at this time. Late in the present study i t became obvious to the author that without an exhaustive study of all structures a report of the complete distribution of the arteries in the median region could not be given, and since no two anomalous embryos would neces- sarily be alike, it has not seemed advisable to prolong the in- vestigation to that extent. Some arteries are perfectly dis- tinct, others are intermingled in arterial complexes or involved in embryonic plexuscs, therefore some are considered only in a general way. On the normal side of each head component, with which the median region must be compared, it is equally difficult at times to thread the dominant pathway through an embryonic plexus, and one cannot safely carry over an in- terpretation from human relations, for in pig many branches of the internal maxillary artery normally take over a large part of the territory served by the external maxillary in man. I n pig the external maxillary does not extend upon the lateral surface of the face.

However, enough principal arteries of the median region of the teras have been determined to warrant a record of tlie findings, which a re presented with full knowledge of their in- completeness. The reader is reminded of the necessity of keeping in mind that some structures in this median area are separate and distinct for their respective head moiety, others contain contributions from both head members, i.e., are con- joined.

Earlier in the paper attention was called to the riglit es- ternal carotid of head B which turns medianward, and running along tlie midplane of the teras, becomes a median external carotid supplying lingual branches to both head components (fig. 1). Whether it is ‘conjoined’ or not cannot be stated positively from this embryo, although inferentially it is fairly safe to assume that in earlier development an arterial plexus

ARTERIAL SYSTEM O F A TWO-HEADED P I G E M B R Y O 217

existed from wliicli the artery in question emerged as the dominant one at the expense of a left external carotid of head A which would lie in close apposition. After giving off the lingual branches, i t continues rostrally in the midplane and passes directly tlirough the median conjoined submaxillary and sublingual glands, giving to them small irregularly branching rami on its way through (figs. 1 and 2 ; '21, figs. 18 and 19). This establishes the artery in this portion of' its course as a median conjoined external maxillary. It then emerges into the less cramped but more topographically dis- turbed areas of the juxtaposed head components, and beneath the conjoined glands breaks up into a plexus from which a pair of slender arteries pass ventrocaudad in the sections (mor- phologically dorso-lateral) to the inner ends of tlie conjoined cartilages of the external auditory meatuses of the left half of head A and the right half of head B (figs. 1 and 2). A t this point they are involved in a fine plexus, out of which they make their way and pass around the conjoined cartilages to the ventral surface of the embryonic head, giving off fine rami into mesenchyme. This corresponds to the area around the external auditory meatuses of the outer sides, hence by in- terpretation from normal pig anatomy, the arteries involved would be chiefly superficial temporals, although owing to the crowded relations the>- might easily include branches from the internal maxillaries of the two head members (figs. 1 and 2) . Traced anteriorly, the pair of slender arteries referred to above come into relation with a plexus common to tlie inferior alveolar arteries of the median region, which will be described presently, and also with an arterial complex associated with the conjoined infra-orbital area (fig. 2 ) .

The inferior alveolar arteries are normally large in the pig and by several mental rami take the place of an inferior labial. These mental rami of the teratological embryo were traced inward through the Meckel's cartilage of each head compon- ent and then as inferior alveolars followed toward their ex- pected origins from internal maxillary arteries, but they

218 MABET, BISHOP

could be traced only a short distance before becoming lost in the mesenchyme owing to their diminutive size, although anastomosis with the linguals is clear. About half-way be- tween symphysis mentis and the otic ends of the Meckel’s cartilages the inferior alveolars were again picked up in the sections and traced backward toward their origins, but in this specimen bone anlage has not yet begun to incorporate them as it has in the 25-mm. normal specimen with which it was compared. Gradually becoming smaller in caliber, they run parallel with and lateral to their respective Meckel’s carti- lages (left of head A and right of head B) , between the car- tilage and the masseter muscle, all the way to the conjoined otic ends where as fine vessels they a re still traceable through a few sections between the conjoined cartilages and the cle- veloping pterygoicl muscles before becoming so obscured that their respective point of origin from an internal maxillary could not be identified (fig. 2) . Part way along their course they come into relation with a plexus common to the arteries clescribed for the region around the conjoined cartilages of the external auditory meatuses (fig. 2) . Several other small arteries ramify symmetrically into mesenchyme of each licacl moiety.

A median stapedial artery has been accounted for on page 213. When followed rostralward after i t passes through the conjoined cartilages of tlie internal ear capsules (fig. 3; ’21, figs. 14 and 15) and on the spinal side of the median geniculatc ganglion, this artery divides into two slender branches which continue rostro-caudad between the muscle masses around 1 he conjoined malleolar cartilages (fused otic ends of Meckel’s cartilages) and the upper pharynx and nasal cavities of tlie two head members and there fade out in mesenchyme (fig. 3) . Near its origin from the median stapedial, each artery gives off a branch which forms a plexus betmTeen the midline of the embryo and the nasal cartilage of each component. A slcnder artery accompanies the inf ra-orbital nerve from the median geniculate ganglion to the snout of each head membw wljich

ARTERIAL SYSTEM OF A TWO-HEADED PIG EMBRYO 219

compensates in part the superior labial and lateral nasal arteries as in the normal pig (fig. 3) .

At the point of formation of the plexus the two divergicg arteries from the median stapedial send two other branches upward which unite in a single vessel in the center of the in- fra-orbital region. Here it forms a delicate plexus from which fine vessels reach out sidewise toward the morplio- logical axis of each head component. The plexus is elon- gated along the midplane, extending into the narrow space between the two adjacent median eyes (i.e., juxtaposed tem- poral regions) (fig. 1). I t is evident from the model and the morphological relations in the microscopical sections that the arteries involved liere a re chiefly branches of the inferior maxillaries of both heads A and B, which, owing to the di- minished area for normal distribution, have become pretty badly crowded, although wherever possible branches assert their normal distribution. They are not in reality continua- tions of the median stapedial artery, as might appear from the model and the diagram made from it, but the stapedial after passing through the conjoined internal ear capsules has become inextricably intermixed with the internal maxillary from each component. Hofman, studying the development of head arteries in the pig with special reference to the stape- dials, calls attention to the fact that the stapedial by its in- ferior ramus normally takes par t in the formation of the in- ternal maxillary. Probably by an exaggerated condition of some such earlier arrangement in the teratological embryo the present condition arose (fig. 1).

The left internal carotid of head A and the right of head B are clear and definite, as also the ophthalmic artery and its ciliary and central retinal artery already described for each head component (p. 211) although belonging to this median region.

A left ethmoidal artery to the nose of head A and a right ethmoidal to the nose of head B can be followed without dif- ficulty, but the origin from their respective oplithalmic

220 MABEL BISHOP

arteries is obscured by the superior oblique muscle of left and right median eyes, respectively. Fo r par t of the way each ethmoidal artery accompanies its nasociliary nerve and is identical with the picture in the normal sides of the terato- logical head.

SUMMARY AND CONCLUSIOS

In summary, the principal regulatory adjustments are : 1. Origin of right and left external carotids in each head

component from a single common carotid of the teratological embryo (fig. 1).

2. Origin of the left lingual artery of head A and the right lingual artery of head B from a median external carotid de- rived chiefly from the right external carotid of component B

3. A median componental external maxillary artery sup- plying the conjoined terminals of the median submaxillarj- and sublingual glands (figs. 1 and 2) . 4. Formation of a basilar plexus from ventral spinal

arteries derived from both head members (fig. 1). 5. Origin of a Y-shaped basilar artery from the basilar

plexus, one limb of the Y passing to head A, the other to head B (fig. 1).

6. The consequent blood flow into the basilar artery from ventral spinal arteries instead of from internal carotids by their posterior rami, as in normal pig.

7. A componental stapedial artery replacing a separate stapedial for the right half of head B (fig. 1).

8. A general mingling of branches of the left internal maxillary artery of head A and the right internal maxillary of head B, both being intimately connected with the median conjoined stapedial artery (figs. 1 and 3).

In conclusion, then, a careful study of the arterial distri- bution in this teratological head shows almost perfect sym- metry and normal, o r near normal, morphological relations whenever the anatomical conditions permit, and disturbed

(fig. 1).

ARTERIAL SYSTEM OF A TWO-HEADED PIG EMBRYO 2’31

topography only where the teratological conditions render it necessary temporarily, but always a return to the normal as soon as the disturbed area is passed through.

The study therefore confirms the earlier investigation of regulatory adjustments in the nervous system, for in both, the embryological and morphological relations are clearly demon- strable, and whenever normal relations are possible, or ap- proximately possible, they assert themselves in convincing manner.

In bringing this last investigation of terata to a close, it may be well to mention that the models of the nervous and arterial systems of this embryo have been given to the em- bryological collection of the Department of Anatomy, Uni- versity of Chicago, but the microscopical sections have been returned to their owner, Dr. II. H. Wilder, Department of Zoology, Smith College, Northampton, Massachusetts.

To Doctor Wilder I wish to express very deep appreciation of the long loan of the embryo that has furnished the material for two of the three teratological studies, and has made it possible to place in the literature the first record of a detailed investigation of two of the earliest systems to begin develop- ment and to compare their behavior as they have adjusted to the physiological needs in a bilaterally symmetrical dicephal- ous pig embryo of 22 mm. length.

I desire also to acknowledge professional courtesies ex- tended to me by Dr. Geo. L. Streeter, of the Laboratory of Embryology, Carnegie Institute of Washington, and to Dr. G. L. Bartelmez, of the Department of Anatomy, University of Chicago, who has given graciously both time and attention to most of the findings and interpretations reported in this paper. To Dr. C. Judson Herrick of the same Department I am also indebted for departmental courtesies and for kindly reading of the manuscript.

222 MABEL BISHOP

BIBLIOGRAPHY

1914 BACH, L., AND SEEPELDER, R. Atlas zur Entwieklungsgesehichte cles men- sehlichen Auges. Leipzig und Berlin.

BISHOP, MABEL Heart and anterior arteries in monsters of the dieephalus group. 1921 Jour. Conip. Neur., vol. 32, pp. 379-429. 1923 Regulatory adjustments in arteries of a dicephalous embryo. Proceedings of the Amer. Assoe. of Anatomists, 39th Session. Anat. Rec., vol. 25, p. 105.

1908

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