STUDIES ON DEGENERATING SEX CELLS IN IMMATURE MAMMALS

N. ABORTIVE MEIOSIS IN SIX STANDARD

STRAINS OF MALE WHITE MICE

EZRA ALLEN Biology Department Stetson Uniwersity DeLand, Florida

TWENTY-FIVE FlQURES

INTRODUCTION

This study was undertaken to provide a complete descrip- tion of the prepuberal differentiation of the male germinal epithelium in the white mouse. I have found no reference to such an account in the literature. Spermatogenesis in white mice has been described by Cox ( '26)' Cutriglit ( '32), Kirkham ('16), Makino ('41)' and Yocum ('17). None of them treats degeneration or the minute details in the structure of the prepuberal germinal epithelium. Early in my study of the germinal epithelium in the Rockland strain of white mice a puzzling mode of degeneration appeared. Atypical small dark stem cells were present in the central plasm of most cords at 7-8 days of age. I have found no reference to this phenomenon in the literature. It is different from the prcpuberal modes of degeneration described by Bryson ('44) in white mice and by Allen and Altland ('52) in normal white rats. Since these small dark cells were discovered not to form spermatogonia, the original plan for this paper could not be carried out. Consequently the history of the unique atypical cells forms the major part of the study.

MATERIAL AND METHODS

A private colony was started in 1952 with the Rockland strain of Swiss white mice. Atypical maturation conditions

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172 EZRA ALLEN

appeared upon histological examination of the testes. Five more strains were added successively as the germinal epith- elium in each proved similarly affected. There were three standard strains from the National Institutes of Health, A, C3H, and BALB/C, with some crosses; the Webster strain; Stocker, and Stocker crossed with Webster, 6 strains in all. They all raised large litters up to the third generation, when the colony was discontinued after 4 years. There had been a few cases of breast tumor, a few of testicular and epididymal tumors ; two mice had flaccid paralysis of the hind legs. Ex- clusive of these, 146 specimens were used in the study.

The mice were sacrificed usually at about 10 A.M. They were aged at one-day intervals from birth up to 21 days of age; after that at longer intervals up to 50 days when typical sperm was found in the vas deferens. The gonads were prepared by standard cytological methods. Helly 's fluid gave the most satisfactory general fixation. It proved best for the chromosomes also up to 15 days; after that age it clumped the chromosomes. Sections were cut at 5,10, and 15 p, stained by hematoxylin or by the Feulgen reaction; some were coun- terstained with fast green. No histo- or cytochemical methods were employed. One 17-day embryo from a former study was used for comparison.

OBSERVATIONS

Structure of the germinal epithelium

At birth the sex cords present two well differentiated types of cell: stem cells and the so-called primordial sex cells. The latter lie scattered in the finely granular central plasm (fig. 2). In figure 1 three are shown at a higher magnification, T and D. Typically they are spherical or ovoid in shape, larger than the stem cells, with three or more nucleoli, and without a visible nuclear membrane. The nucleoli stain deeply with hematoxylin but faintly with Feulgen. After birth these cells have no visible cytoplasm, but in the 17-day embryo cytoplasm appears in many of them. They have not been found

ABORTIVE MEIOSIS IN MALE WHITE MICE 173

in mitosis in the 17-day embryo or after birth. They are present in nearly every section of the 1-day cord (figs. 1 and 2) , but at three days only a few appear ; most of the cords have none (fig. 3) . At 5 days very few were found. In degeneration the nuclear plasm and nucleoli become pale and are resorbed in situ (fig. 1, D).

The stem cells at birth form a single layer one cell wide supported on the laminated connective tissue basement mem- brane of the cord. For the most part they are similar in size and structure having no visible cytoplasm ; in shape, spherical or ovoid. They present the same appearance after hematoxylin or Feulgen treatment. The nuclear plasm is composed of a finely granular homogeneous substance containing 3-5 deeply staining nucleoli (figs. 1 and 2 ) ; a well defined nuclear membrane is present. Stem cells are recognizable by these characteristics throughout the normal life of the testis.

Early differentiation of the stem cells Two types of stem cell are differentiated early: narrow

flask-shaped cells and “large” cells (figs. 4 and 5). The stems of the flask-shaped cells rest on the basement membrane, the bowls extend well into the central plasm. They are not numerous. First seen at two o r three days after birth; rarely after 10 days of age. A nuclear membrane is present and several nucleoli ; no cytoplasm is evident. Apparently they are not specialized for a definite function. The inner struc- ture of the bowls is the same as that of the accompanying stem cells. The shape is probably due to lateral crowding of adjacent stem cells during growth of the cord. They have not been seen in mitosis. Allen and Altland (’52) report them in the early prepuberal stages of the white rat.

The “large” cells (fig. 5) were first described by the writer in his study of the early prepuberal germinal epithelium of the white rat (Allen, ’49), and named “large” from their exceptional size. The “large” cells in the mouse testis are essentially similar, and have the same distinctive mode of degeneration and resorption. The other stem cells are dis-

174 EZRA ALLEN

tinctly smaller in division than the same stages of the “large” cells, and the chromosomes are usually less discrete. Compare figures, 5 and 6 made under the same magnification. The “large” cells may divide by mitosis up to anaphase and degenerate, or they may begin to degenerate at metaphase. The chromosomes break up into variously shaped bodies which become smaller and smaller, finally disappearing in a nearly amorphous plasm (fig. 3, D). Some cytoplasm may persist. At prophase they lie in the basal layer of the epithelium, but later may push into the central plasm. Very few were found after 10 days. The cell in figure 5 shows 30 chromosomes; in another, 34 were counted. The diploid number in the mouse is accepted as 40.

Fur ther stages of differentiatiofi

In order to make clear the atypical maturation process due to the presence of the small dark cells noted in the introduction, it is necessary to review briefly the generally accepted typical maturation series of the stem cells into mature sperm in mammals. This is as follows: spermatogonia, first spermatocytes including the chromosomal differentiations into leptotene, zygotene, pachytene, diplotene, diakinesis, first and second spermatocyte division ; spermatids, mature sperm. Any structural or temporal change from this order of events is regarded as atypical.

A preliminary series of developmental changes precedes the series just described. These are as follows: the single- layered epithelium widens by mitosis, and some cells a little larger and more deeply stained appear. Then the latter cells move outward and form a separate layer. Some of them at a certain age, depending on the species, differentiate sper- matogonia and thus start the typical maturation series. The spermatogonial cells are also called B cells, and the stem cells A cells. See figure 19. The spermatogonia (B cells) divide mitotically and their daughter cells differentiate into leptotene cells, which in turn develop the zygotenes, these

A. Typical.

ABORTIVE MEIOSIS I N MALE WHITE MICE 175

the pachytenes, etc., until the sperm are produced. Thus, by repetition, a constant supply of sperm is assured during the sexual life of the individual.

In the 6 strains of white mice studied, the preliminary changes began at 5-6 days of age iry a few cords, and at 9 days the 2-layered epithelium had developed (fig. 7). The cells of the inner layer had maintained the characteristic stem cell structure. Up to this age there is nothing atypical in these few cords.

B. Atypical. His tory of the small dark cells. However, a different picture is presented in most cords. At 7-8 days (5 in one specimen, fig. 8) small dark cells had begun to appear among the stem cells, usually central to the single epithelial layer. This mode of origin in the cord is seen especially clearly in figure 9, in a 10-day cord, because they have taken the stain less deeply than generally. Two or three nucleoli appeared but no visible cytoplasm. In size, they average about the same as the other stem cells. Judging from figure 9 one might conjecture that they arise by mitosis of the stem cells, but further evidence indicates that they arise by differentiation. Certainly no large number of mitotic cells has been seen previous to their appearance, or among them. Hereafter the small dark cells will be conveniently called SDCS. In figure 10, made at 11 days of age, another mode of disposition of the SDCS as they arise is seen; they form a complete basal layer lined with a layer of typical stem cells.

By either the first or the second mode of distribution, many cords had become filled with SDCS at 13 days (fig. 11). Many cells are apparently in an early stage of reorganization com- pared with the newly formed SDCS in figure 9. The under- lined cells when enlarged (fig. 12) indicate change in nucleoli together with a general distribution of the chromatin. Many cells in figure 11 have still retained one or more nucleoli. Further reorganization is seen in figures 13 and 14; the underlined cells in figure 13, enlarged in figure 14, have developed some atypical chromosomes. (Ab.) Most of the central cells in figure 13 are so deeply stained that internal

176 EZRA ALLEN

bodies may be dimly discerned only by the aid of a hand lens. In neither figure 13 nor 14 is there evidence of typical mitosis. If division is occurring it is atypical.

Several other types of cord were present at 13 days: Some had developed a lumen surrounded by cells similar to those in figure 13. A very few cords had differentiated only to the preliminary 2-layered stage like figure 7. Some were dif- ferentiating incompletely into leptotenes, figures 17 and 18. In figure 18 some central cells show typical leptotene chro- mosomes (T), but most other central cells have an abortive structure ; in 17, the basal eipthelial cells appear to be incom- pletely developed B’s. On the other hand a very few cords were in the typical leptotene stage of the maturation series like figure 19. A few cords were similar to figure 15 in which the underlined cells are in the typical position of early leptotenes, but their structure is abortive (fig. 16) ; the outer cells are still SDCS instead of B’s and A’s which typically accompany such leptotenes. Compare with figure 19.

At 14 days, intermingled cords and tubules were present in increased variety. All those described at 13 days had con- tinued. I n addition, some had one to three basal layers of SDCS and no other types of cell; several tubules had abortive zygotenes and abortive basal B’s. Many tubules had well advanced abortive leptotenes. One tubule had two layers of typical leptotenes inside two layers of stem cells. Most other tubules of that specimen had two layers of SDCS with central abortive leptotenes. In others, several tubules with abortive zygotenes or typical diplotenes in the epithelial layers were present. The most advanced typically had a basal layer of B’s, A cells next, and zygotenes in center (fig. 20). There were not very many such typical cords. Although no counts were made the ratio between the typical and abortive cords is approximately shown in figure 21.

During the period between 15 and 17 days a well advanced lumen was generally present. The very young 2-layered cord (figure 7) was found only a t 15 days. Otherwise during this period the variety of tubules was the same as at 14 days,

ABORTIVE MEIOSIS I N MALE WHITE MICE 177

with the addition of some with degenerating abortive zy- gotenes. The number of typical tubules had increased some- what. Apparently the deleterious influence of SDCS was becoming less potent.

At 18 days most of the abortive conditions continued, but the typical stage shown in figure 20 had increased in number ; there were also some tubules with central leptotenes instead of zygotenes, similar to figure 19. There were a few typical and abortive dividing spermatocytes and typical and degen- erating spermatids. Some SDCS had persisted. Many tubules had basal A’s with central abortive leptotenes.

The later stages are summarized. 19 to 33 days. In this period there is still great varia-

tion in development. No SDCS were found after 20 days. Their disturbing effects at previous ages, however, were manifest throughout this period in various abortive stages. Some specimens were entirely typical up to the stage shown in figure 20. At 19 days two specimens had sperm heads in the epithelium (fig. 22), though there were also many other atypical tubules. Degeneration of abortive zygotenes con- tinued, with the addition of degenerating spermatids and sperm heads inside the epithelium,

34 to 50 days. At 34 days no atypical conditions were found except some degeneration of spermatids and sperm heads. In the older stages typical maturation stages continued. Typical sperm was present in the epididymis in one specimen at 41 days; in another at 50 days in both epididymis and vas def erens.

DISCUSSION

The “large” cell (figure 5) . This cell so closely resembles one I described in the prepuberal epithelium of the white rat (Allen, ’49) that I believe they have the same origin- from stem cells. Bryson (’44) in figure 7 of his paper on the white mouse shows a similar metaphase view of a large cell and a prophase view of a large cell, both in the epithelium. These he labels “r” cells and interprets them as primordials.

178 EZRA ALLEN

I cannot agree with him. I have no evidence of dividing primordial cells in the white rat or in the white mouse in either the prepuberal stage or in the 17-day embryo. I n papers by previous workers the primordial cells have been located only in the central plasm. In his figure 7, Bryson shows also a small nucleus in the central plasm, which I would interpret as a degenerating primordial. Perhaps cytochemical studies may aid in understanding this unusual cell. It seems to be a special cell, but with no observed function.

Vuriation. I n the 6 strains studied, variation is common among the different developmental stages in the same in- dividual and in different individuals of the same age. It occurs in the number of primordial cells from one to 6 days of age ; in the age of formation of the two-layered epithelium ; in the number of flask-shaped and “large” cells; in the number of small dark cells (SDCS), a t ages 5-20 days after birth; and in the number of the different kinds of cords and tubules from 11 days to 33 days.

A few exceptional cases are notable as examples of extreme variation at the same age. Two 19-day specimens had de- veloped typically up to sperm heads in the epithelium (fig. 22). In contrast, another of the same age had a wide range of variation, including numerous tubules with SDCS, typical and abortive leptotenes and zygotenes, abundant abortive cells (fig. 24), degenerating abortive dividing spermatocytes, and degenerating spermatids. In another specimen, at 18 days, the left gonad had no SDCS, but typical meiotic stages up to dividing spermatocytes, the most advanced stage present ; in the right gonad one part had numerous SDCS and un- identifiable abortive cells, yet the rest of the tissue was typical up to the pachytene stage.

Several modes of degeneration in the pre- puberal stages of development have been described in mam- mals, including man. Commonly, individual cells pass into the lumen; o r layers of inner cells degenerate by exfoliation; or more rarely part or all of the epithelium in some tubules suffers complete necrosis. The 6 strains of white mice de-

Degeneration.

ABORTIVE MEIOSIS IN MALE WHITE MICE 179

scribed in this paper are unique in their chief mode of degeneration. Small dark stem cells (SDCS) which first ap- peared in small numbers in most of the cords from 7 to 9 days of age, located centrally or in a complete basal epithelial layer, increase rapidly in number until at 13 days they may fill most of the cords without having formed spermatogonia (B cells). They differentiate into abortive leptotenes and zy- gotenes and degenerate. No further abortive stages of the SDCS have been found. Degeneration takes place as follows : nuclei and cytoplasm become less dense and are finally re- sorbed ilz situ. Degenerate typical or abortive germ cells were not found in the rete ; or in the epididymis except in one mouse, aged 26 days. The early spermatide degenerate singly ; in the older stages they pass into the lumen in small groups. Some abortive dividing spermatocytes and some sperm heads also degenerate. After the age of 34 days exfoliation and necrosis were observed in a few tubules. Typical sperm was first found in the epididymis at 34 days.

These cells did not interfere with the health or the body growth of the affected mice, or the increase in the size of the tubules; compare figures 7 and 22 made at the same magnification. No loss of fertility oc- curred except in a very few mice that had either tumors or flaccid paralysis of the hind legs. Large litters appeared continuously in the colony up to the third generation; no fourth generation was tried.

The positive effects observed were: (1) The SDCS dif- ferentiated no spermatogonia at 10 or 11 days or later. The exact age when they should appear was not determined. Kirkham ( '16) reports spermatocytes in 13-day mice ; Bryson ('44) at 10 days. The deleterious effects were overcome in a few cords which had reached the typical zygotene stage at 14 days (fig. 20). Most 14-day cords had no typical cells.

During the period between 15 and 18 days there were many tubules with large basal cells like A's that were apparently inhibited from differentiating into B 's ; abortive aygotenes filled the central part of the tubules, which in-

The small dark cells.

(2)

180 EZRA ALLEN

dicated that they interfered with typical meiosis of the A cells, In one 19-day specimen basal layers of SDCS had apparently caused the abortive division of spermatocytes and, with few exceptions, the failure of spermatids to develop.

As to the cause of the SDCS one may only conjecture. Perhaps it is genic. It would seem quite possible that all the strains of white mice now used for experimental purposes in America originated from one original Swiss strain. If it still exists we do not know whether it possessed SDCS. Environmental conditions such as food and housing might be the cause. Another possible cause might be the age at which breeding occurred. I n securing my mice I asked the National Institutes of Health and the different commercial sources to send me mice old enough to breed. They did not give me the age in days. I n my colony I let the mice so secured begin breeding without waiting for more mature development. In the colony, mothers were left with the litters until about 20 days of age, and then removed and re-mated. They were surely not immature. The young males and fe- males in the litters were not separated until pregnancy ap- peared, so that they might have begun breeding while rather young. I have been told by a breeder of hamsters that a variety of abnormalities appear if breeding is started too young.

There are several factors now know which may account for the satisfactory fertility in these mice in spite of the great loss and delay due to the SDCS. The first three factors named are conditioned on the survival of the affected individuals. (1) The 2-layered early cords like figure 7 present from 13 to 15 days might differentiate typically. (2) Likewise, we may assume that the typical 14-day zygotenes shown in figure 20 would complete their maturation. (3) Many 15-20 day tubules have abortive central zygotene cells with large basal A cells which are apparently about to produce B’s. Resorption of the atypical eygotenes would enable the basal cells to complete their differentiation into B cells. Such regeneration of a complete germinal epithelium after the

Fertilitg.

ABORTIVE MEIOSIS IN MALE WHITE MICE 181

necrosis of the inner layers of cells is known to have occurred in other mammals. Finally the disappearance of the abortive cells at 34 days shows that the apparent struggle for germ cell maturation has been won and later development may meet with no opposition.

SUMMARY AND CONCLUSIONS

The study was undertaken to provide a complete detailed description of the prepuberal germinal epithelium in the male white mouse. A total of 146 healthy mice from 6 standard strains were used. All the strains grew well and produced large litters. At 9 days of age some of the stem cells had completed the formation of the 2-layered epithelium, but only a few cords later produced spermatogonia from the outer layer. In most cords some small dark cells among the stem cells at 7-10 days did not differentiate spermatogonia, but did produce abortive leptotene and zygotene meiotic stages.

The small dark cells multiplied rapidly until at 13 days most of the cords were filled with them. At 14 days a few cords at the typical zygotene stage appeared intermingled with a variety of abortive leptotene stages. Some of the cords were at the early two-layered epithelium stage up to 15 days. No small dark cells were found after the age of 20 days, but their abortive stages continued to 33 days. Atypical cords and tubules mingled with a few typical ones continued up to 33 days. Many of the atypical tubules contained degenerating zygotenes, spermatids, and sperm heads within the epithelium. At 34 days practically no atypical conditions were present, and typical maturation continued until sperm was found in the vas deferens at 50 days.

Sertoli cells appeared in one specimen at 14 days. It is concluded that the small dark cells lack the potency

to develop typical meiosis. The successful fertility of these mice in spite of the great loss of cells by degeneration may partially be explained by certain factors in the history of the maturation stages, if the animals should be permitted to survive to maturity: (1) The possible continued differentia-

182 EZRA ALLEN

tion of the cords in the stage of the 2-layered epithelium. (2) From 15-33 days many tubules were present with basal cells almost at the B stage, but the central cells were degen- erating. Regeneration after loss of part of the prepuberal epithelial cells in some mammals has been cited on p. 181. (3) The typical zygotene cells present at 14 days and older ages might also produce later maturation stages up to viable sperm. Finally, these contributions, though not large, might have a potent influence against the early development of the small dark cells among the residual stem cells.

LITERATURE CITED

ALLEN, E. 1949 Studies on degenerating sex cells i n immature mammals. I. An analysis of degeneration in primordial and large germ cells in male albino rats aged 1-9 days. J. Morph., 85: 405-422.

ALLEN, E., AND P. D. ALTLAND 1952 Studies on degenerating sex cells in immature mammals. 11. Modes of degeneration in the normal dif- ferentiation of the definitive germ cells in the male albino rat from 12 days to maturity. J. Morph., 91: 515-540.

BRYSON, V. Spermatogenesis and fertility in Mus musculus as affected by factors a t the T locus. J. Morph., 74: 131-187.

Cox, E. K. 1926 The chromosomes of the house mouse. J. Morph. and Physiol., 43: 45-54.

CUTRIGHT, P. R. 1932 The spermatogenesis of the mouse (Mus musculus var. albula). J. Morph., 54: 197-220.

KIRKHAM, W. B. 1916 The germ cell cycle in the mouse. Anat. Rec., 10:

MAKINO, S. 1941 Studies on the murine chromosomes. I. Cytological in- vestigations of mice included in the genus Mus. J. Fm. Sci. Univ. Hokkaido, Zool., 7 I 305-380.

YOCUM, H. B. 1917 Some phases of spermatogenesis in the mouse. Univ. Calif. Publ., 16: 371.

1944

2 17-2 19.

PLATES EXPLANATION OF PLATES

All figures are photomicrographs hp the writer, made a t the magnifications stated and not reduced in publication. The sections were stained with hematoxglin and the Feulgen reaction ; nisny were counterstained with fas t green. The magnifications were obtained with a Bausch and Lornb 4.3 fluorite oil immersion objective, and either a 5 x Leitz planar or a 10 X I A t z compensating ocular combined with an adjiist,ablc camera bello\vs.

PLATE 1

EXPLANATION OF FIGURES

1

2

3

8

9

A 1 d:ip priinorclinl nnd 2 dt~geiierating primordial cells, D. X 2000.

A 1 (lay cord with several primordial cells. X 1000. Note the 1-laycreu epitlirliuiii in figures 1, 2, :lnd 3.

Two 3 day corcls without primordid cells. X 1000. These cords are like most of the 3-tlny cords; :I few cords still have some primordials. The cell marked I) in the upper srctioii is a degenerating “ largc ’ ’ cell.

Xcveral flask-shnpetl epithelial cells projecting central; a t 5 days.

A Lcla rge” cell in iiirtnpliahe; n t 3 (laps.

,4 stcm cell n t i i ietnplmr; a t 10 days. X 2000.

.\ 9-day rortl with %-layered epithclinrn. X 1000. Note differericcs between inner :iiicI outrr layers.

Two 3-tl:iy cords sliowiiig small dark cells (SDCS) central. x 1000. This is tlic only 5 clay spccimrii found that had produeed the SDCS.

A 10 diiy cord with 2-lsyerc~il epitlielium and SDCS rentrnl; also cells iii

X 2000.

X 2000.

r of epithelium. x 1000.

ARBREVTATTONS

A, A erlls Al), :tbortive E, B cells I), drgcncrate or tlrgcncrating

F, flask-shaped cell Sp, spermatids T, typic,al

184

PLATE 1

185

PLATE 2

EXPLANATION OF FIGURES

10 Part of an 11-day cord. Basal layer of SDCS one cell wide and an inner lnyw of stem cells. X 1000.

11-18 Tlirsc figures show wrious representative conditions in 13-day cords. The figures on thr left side a r r photomicrographs made at X 1000; the cells undrrlinecl arr inngnifird about 33 times i l l the figures on the right side.

11 and 1 2 'I'hc cells in this cord show a greatly increased number compared with those in figure 9, and n slight reorgaiiization of the chromatin w1iic.h nppe:rrb distinctly in the enlarged cells, figurc 12.

13 ;ind 14 The uiiderlincd cells in this cord show abortive cells; All, fignrc 14. Cornpii~e with typical chromosomes in figure 18, T.

15 and 16 The structure of this cord is difficult to intrrpret. Tlic 1;irge central uiiderlinrcl cells are in tlie typical position of enrlg leptotcne cells of the maturation series, but the two or tlirec layers of outer cells are still SDCS instead of B a i d A cells as in figure 19. Also the chromosornex, eiilnrgetl i n figure 16, Ab, a r e not tgp icd leptotenes likc those shown in figure 18, :is may be seen i n figure 15, if rramined with a hand lens.

17 :ind 18 This cord s I io \~s an iiicorriplete leptotcne stage. While the clrromosomrs i n two of the three cells uiiderlinrtl in figure 17 and enlnrgetl ill figurc. 18 are typiral prop1i:iw leptotent>s, the ccll I)rtwecn thrsr two is :rl)or.tivc. The cells in tlie 1~ : i~a l layer (fig. l i ) arc not :w coinplctcly cliff'ereutinted ns a r r the bns:rl cells 111 flgllle 20.

19 A normal white rat cord introduced to show the structure cipeeted in a typical white mouse or' com]iar;itivr age - lxisal Inyrr R 'b, inricr la ye^ A' \ , and central cells leptotenes. X 1000.

A 14-day typiciil coril of one of the 6 white mice: h:is:rl layer R's, i 1 1 1 i n

layer A's, central cells zygotenes. 20

X 1000. (Compare with figure 19.)

20n Foul cells from H cord i n the same seetioil :is figurr 20. X 2000. Thesp :ire clearly xpgotenrs. Notv t l i c b nucleolus and X chromohomr in tlw upperniobt cell.

187

PTAATE 3

EXPLANATION OF FIGURES

21 Several cords from the sainc sections as figure 20 showing two typical cords, TI and many abortivr cords. X 550.

22 Typical differentiation in a. 19-day specimen : cells in diakinesis, dividing first spernmtocytes, a few degcncratirig spermatids, Sp and D, and typical sperm heads differrntiating within the epithelium. X 1000.

23 Mature sperm discharging into the lumen; adult. X 550.

24 Section of tubnle in a 19-di1y inouse. The SDCS form the basal lager of the epithelium; a few stcm cells in the next layer; the center is filled Tvith abortive cclls, some plainly showing degeneration.

A tubule from the same section as figure 24 showing advanrcd degeneration stages along the lumen, D. X 100. Most of the tubules in this specimen slioivctl mnre or less degeneration.

X 1000.

25

AUORTTVI': MEIOSIS I N MALE: WHITE MICE R X R I ALLES

PLATE 3

189