Name of presentation The branch of biology

that deals with the formation, early

growth, and development of living

organisms. Descriptive Descriptive

embryologyembryology Comparative Comparative

embryologyembryology Experimental Experimental

embryologyembryology Chemical Chemical

embryologyembryology TeratologyTeratology Reproductive Reproductive

biologybiology Developmental Developmental

biologybiology •• Concerned with Concerned with

understanding the understanding the basic structural basic structural pattern of the pattern of the embryonic bodyembryonic body

•• Tools: makes use of Tools: makes use of techniques in techniques in serialserial --sections and sections and of making of making threethree--dimensional dimensional wax plate wax plate reconstruction.reconstruction.

•• Compares the Compares the

development of one development of one species with that of species with that of anotheranother

•• Provided insight to Provided insight to the concept that the concept that ““ontogeny ontogeny

recapitulates recapitulates phylogenyphylogeny”” –– All vAll vertebrate embryos ertebrate embryos

follow a common follow a common developmental path developmental path due to their common due to their common ancestry. All have a ancestry. All have a set of very similar set of very similar genes (the homeobox genes (the homeobox genes) that define genes) that define their basic body plan. their basic body plan. As they grow, the As they grow, the differences that will differences that will distinguish the distinguish the embryos as adults embryos as adults becomebecome more and more and more apparent. more apparent.

•• Provided Provided

descriptive descriptive information about information about the chemical and the chemical and physiological physiological events in an events in an embryoembryo

•• Concerned with the Concerned with the

study of study of malformationsmalformations

•• Deals with Deals with

practically oriented practically oriented problems involving problems involving techniques of techniques of ferti l ization and ferti l ization and contraception in contraception in both humans and both humans and domesticated domesticated animals.animals.

•• It puts heavy It puts heavy

emphasis on emphasis on normal normal gametogenesis, gametogenesis, endocrinology of endocrinology of

reproduction, reproduction, transport of transport of gametes, early gametes, early embryonembryonic ic development, and development, and implantation of the implantation of the mammalian mammalian embryo.embryo.

•• Encompasses not Encompasses not

only embryology, only embryology, but even postnatal but even postnatal processes such as processes such as normal growth, normal growth,

metamorphosis, metamorphosis, regeneration, and regeneration, and tissue repair at tissue repair at levels of levels of complexity ranging complexity ranging from the molecular from the molecular to the orgato the organismal.nismal.

•• It focuses on It focuses on

processes and processes and concepts rather concepts rather than specific than specific morphological morphological structures.structures.

•• Involves the Involves the

conversion of a conversion of a single cell, the single cell, the ferti l ized egg, into ferti l ized egg, into a complex a complex organismorganism

H O W H O W D O E S D O E S D E V E L O PD E V E L O PM E N T M E N T

W O R K ?W O R K ? • Conversion of germ

plasm into highly specialized sex cells (ova & spermatozoa) capable of uniting at fertilization and producing a new being. – SPERMATOGENESIS – OOGENESIS

• Gametes are formed

from germ cells • Future germ cells

become committed to their fate at an early stage of animal development

• In some cases,

cytoplasmic determinants present in the egg programs cells that inherit it to

become germ cells – Associated with a

visible specialization of the cell called the GERM PLASM

– Found in: •C. elegans – polar

granules •D. melanogaster –

pole plasm •Xenopus – vegetally

localized germ plasm rich in mitochondria

• During embryonic

development, germ cells undergo a period of multiplication, then migration

• In mid-development,

sex determination is made

• In post embryonic development, gamete formation is done.

MEIOSIS • Takes place in sex cells • Produces 4 daughter cells

that are haploid • Used to produce haploid

cells • Includes 2 successive cell

division • Includes cytokinesis • Has 4 phases: Prophase,

Metaphase, Anaphase, and Telophase

E A R L Y E A R L Y D E V E L O PD E V E L O PM E N TM E N T

•• a a process, not a process, not a single event, that single event, that begins when a sperm begins when a sperm cell first makes cell first makes contact with the contact with the coverings of the egg coverings of the egg and ends with the and ends with the intermingling of intermingling of maternal and maternal and paternal paternal chromosomes at the chromosomes at the metaphase plate metaphase plate prior to the first prior to the first cleavage division.cleavage division.

•• Important Important components:components:

•• Initial membrane Initial membrane contact between contact between egg and spermegg and sperm ••Sperms come in Sperms come in contact with the contact with the jelly coatjelly coat ––motility is motility is increased.increased.

––acrosome acrosome reaction is reaction is stimulated. stimulated.

•• Important Important components:components: •• Entry of the sperm Entry of the sperm

cell into the eggcell into the egg •• Important Important

components:components: •• Prevention of Prevention of

polyspermypolyspermy by the by the eggegg ••fertilization of fertilization of the egg by more the egg by more than one sperm; than one sperm; ••will result to will result to polypoidy and polypoidy and early disruption early disruption of the development of the development and death of the and death of the embryoembryo

•• Important Important components:components: •• metabolic metabolic

activatactivation of the ion of the

eggegg ••activates a activates a program of events program of events patterned in the patterned in the egg.egg. ••these events these events prepare the egg for prepare the egg for the main event in the main event in fertilization: fertilization: the fusion of the the fusion of the genetic material genetic material from the egg and from the egg and the sperm.the sperm.

5. Completion of 5. Completion of meiosis by the eggmeiosis by the egg

•• Important Important components:components: 6. Formation and 6. Formation and

fusion of male and fusion of male and female pronuclei, female pronuclei, leading to the first leading to the first cleavage divisioncleavage division

•• Leads to the Leads to the formation of a formation of a zygote.zygote.

•• TheThe rapidrapid mitoticmitotic divdiv

isionision ofof aa zygote zygote withwith decreasedecrease inin sizsizee ofof individualindividual cells cells or blastomereor blastomeres and s and the formation of a the formation of a

morula.morula. •• The one cell The one cell embryo undergoes a embryo undergoes a series of cleavage series of cleavage divisions, divisions, progressing progressing through 2through 2--cell, cell, 44--cell, 8cell, 8--cell and cell and 16 cell stages. 16 cell stages.

•• The cells in The cells in cleavage stage cleavage stage embryos are known embryos are known as blastomeres. as blastomeres.

•• Soon after Soon after

development of the development of the 88--cell or 16cell or 16--cell cell embryo (depending embryo (depending on the species), on the species), the blastomeres the blastomeres begin to form begin to form tight junctions tight junctions with one another, with one another, leading to the leading to the formation of a formation of a mulberrymulberry--shaped shaped mass of cells mass of cells called a called a morulamorula. .

•• change in shape of change in shape of the embrythe embryo is o is called called

compactioncompaction. . •• Stage of genetic Stage of genetic maternal effects. maternal effects. –– The properties of The properties of the the cleavagecleavage--stage stage embryo depends embryo depends entirely on the entirely on the genotype of the genotype of the mother and not of mother and not of the embryothe embryo

–– embryoembryo ’’s own genome s own genome remains inactive remains inactive during part or all during part or all of the cleavage of the cleavage phase phase

–– protein synthesis protein synthesis

is directed by mRNA is directed by mRNA transcribed transcribed during oogenesis during oogenesis (maternal mRNA)(maternal mRNA)

•• Patterns of Patterns of Cleavage:Cleavage: 1. 1. Holoblastic Holoblastic cleavagecleavage: occurs in : occurs in isolecithalisolecithal eggs eggs (mammals, sea (mammals, sea urchins). The urchins). The entire egg is entire egg is cleaved during each cleaved during each division.division. 2. 2. Meroblastic Meroblastic cleavage cleavage occurs occurs when eggs have a lot when eggs have a lot

of yolk. The egg does of yolk. The egg does not divide not divide completely at each completely at each division. Two division. Two types:types:

•• Patterns of Patterns of Cleavage:Cleavage: –– Two types:Two types: a. Discoidala. Discoidal cleavage is limited cleavage is limited to a small disc of to a small disc of cytoplasm at the cytoplasm at the animal pole. All of animal pole. All of the yolk filled the yolk filled cytoplasm fails to cytoplasm fails to cleave cleave (characteristic of (characteristic of

telolecithal eggs telolecithal eggs such as birds). such as birds).

•• Patterns of Patterns of Cleavage:Cleavage: –– Two types:Two types: bb. Superficial . Superficial cleavage is limited cleavage is limited to a thin surface to a thin surface area of cytoplasm area of cytoplasm that covers the that covers the entire egg. The entire egg. The inside of the egg inside of the egg that is filled with that is filled with yolk fails to cleave yolk fails to cleave (centrolecithal (centrolecithal eggs such as eggs such as insects).insects).

•• consist of layers consist of layers of blastomeres, of blastomeres, known as the known as the blablastodermstoderm, which , which surrounds a surrounds a cavity, the cavity, the blastocoeleblastocoele

•• In mammals the In mammals the blastula is blastula is referred to as a referred to as a blastocystblastocyst..

•• a significant a significant amount of activity amount of activity occurs within the occurs within the

early embryo to early embryo to establish cell establish cell polarity, cell polarity, cell specification, specification, axis formation, axis formation, and regulate geand regulate gene ne expression.expression.

•• the mid blastula the mid blastula transition (MBT) transition (MBT) is a crucial step is a crucial step in development in development since the maternal since the maternal mRNA is degraded mRNA is degraded and control over and control over development is development is

passed to the passed to the embryo.embryo.

•• CCharacterized by the haracterized by the

morphogenetic morphogenetic movements of cells movements of cells that creates the 3 that creates the 3 gergerm layers: the m layers: the ectoderm, ectoderm, mesoderm, and mesoderm, and endodermendoderm –– Each layer gives Each layer gives

r ise to specif ic r ise to specif ic t issues and organs t issues and organs in the developing in the developing

embryo. embryo. •• Laying down of Laying down of body plan body plan of the of the embryo: region of embryo: region of cells committed to cells committed to become a become a particular body particular body part; not yet part; not yet differentiateddifferentiated

•• Phylotypic stage Phylotypic stage –– stage at which stage at which different members different members of an animal group of an animal group show maximum show maximum similarity to each similarity to each

other.other. •• Radially Radially

symmetrical symmetrical –– unferti l ized eggunferti l ized egg

•• Cytoplasmic Cytoplasmic rearrangement rearrangement creates bilateral creates bilateral symmetrysymmetry –– Occurs after Occurs after

ferti l izationferti l ization •• Change of Change of

symmetry symmetry establishes the establishes the

dorsal an ventral dorsal an ventral s ideside

•• Establishment of Establishment of

meridional planes meridional planes occur after the occur after the formation of formation of bi lateral symmetry bilateral symmetry –– Medial (sagittal) Medial (sagittal)

plane plane –– divides the divides the r ight and the left r ight and the left s ide; often the side; often the plane of the first plane of the first cleavagecleavage

–– Frontal plane Frontal plane –– the the meridional plane meridional plane at r ight angle to at r ight angle to the first; often the the first; often the plane of the 2plane of the 2ndnd cleavagecleavage

–– Anteroposterior Anteroposterior

plane plane (craniocaudal or (craniocaudal or rostrocaudal) rostrocaudal) –– head to tai l axis; head to tai l axis; occurs after occurs after gastrulationgastrulation

–– Dorsoventral Dorsoventral –– toptop--bottom axesbottom axes

–– MediolateralMediolateral –– leftleft -- r ight axesright axes

•• Principal body Principal body

parts become parts become visible after vis ible after completion of completion of gastrulation.gastrulation.

•• Segmentation Segmentation occurs f irst at the occurs f irst at the

anteroposterior anteroposterior axis.axis.

•• Situs solitus Situs solitus –– asymmetrical asymmetrical arrangement of arrangement of body partsbody parts

•• Sets of genes whose Sets of genes whose

expressions control expressions control the state of the state of commitment of commitment of different cells.different cells.

•• The entire group of The entire group of processes that mold processes that mold

the external and the external and internal internal configuration of an configuration of an embryoembryo

•• Encode Encode transcriptional transcriptional factors whose factors whose function is regulating function is regulating the activity the activity of other of other genes.genes.

•• concerned with the concerned with the

shapes of t issues, shapes of t issues, organs and entire organs and entire

organisms and the organisms and the posit ions of the posit ions of the various specialized various specialized cell types cell types

•• Throughout these Throughout these processes, cell processes, cell movements, movements, rearrangements, rearrangements, and shape changes and shape changes contribute to the contribute to the final form thafinal form that t defines the tissues, defines the tissues, organ, or organism. organ, or organism.

•• PATTERN PATTERN FORMATIONFORMATION–– laying laying down of the down of the morphogenetic morphogenetic blueprintblueprint oo Example: Example:

AnteroAntero--posterior posterior axes patterning in axes patterning in Drosophila Drosophila melanogastermelanogaster Morphogenesis Morphogenesis of the of the

fruit f ly Drosophila fruit f ly Drosophila melanogaster starts melanogaster starts with the construction with the construction of asymmetries within of asymmetries within

the oocyte and the oocyte and proceeds to pattern proceeds to pattern formation along the formation along the embryonic axes embryonic axes

•• Process where the Process where the

effect of one effect of one embryonic tissue (the embryonic tissue (the inductor/source) inductor/source) on on another is so that the another is so that the developmental developmental

course of the course of the responding tissue is responding tissue is qualitatively changed qualitatively changed from what it would from what it would have been in the have been in the absence of the absence of the inductor.inductor.

•• Example: formation of Example: formation of eye lens as a result of eye lens as a result of the inductive action the inductive action of the optic cup.of the optic cup.

•• When tWhen the inducing he inducing

factors can i l l icit factors can i l l icit more than one more than one

threshold responses threshold responses that brings about that brings about the formation of a the formation of a complex pattern complex pattern in one step.in one step. –– Gradient controls Gradient controls

the types of the types of terr itories, their terr itories, their sequence in sequence in space, and the space, and the overall orientation overall orientation or polarity of the or polarity of the serser ies of new ies of new structures.structures.

•• To set up a To set up a

concentration concentration gradient, a gradient, a continuous pulse of continuous pulse of morphogens should morphogens should be produced from be produced from a source and a source and destroyed in a sink.destroyed in a sink.

•• 2 important 2 important

properties of properties of concentration concentration gradientgradient

–– Should be able to Should be able to divide the competent divide the competent zone of cells into zone of cells into several states of several states of commitment by commitment by means of threshold means of threshold responsesresponses

–– It should be able to It should be able to impart polarity and impart polarity and pattern to the pattern to the responding tissueresponding tissue

•• One that can One that can

convert one body convert one body part into another.part into another.

•• May happen if:May happen if: –– a 2 a 2ndnd gene in the gene in the

series cannot be series cannot be turned on. In this turned on. In this case, the 2case, the 2ndnd body body segment wil l segment wil l become a duplicate become a duplicate of the 1of the 1 stst body body segment. Fate of segment. Fate of the 3the 3 rdrd segment segment cannot be cannot be predicted predicted (loss(loss --ofof -- function function homeotic mutation); homeotic mutation); wil l produce wil l produce

anteriorizatanteriorizat ionion •• May happen if:May happen if:

–– a 2 a 2ndnd gene in the gene in the series is always series is always turned on. In this turned on. In this case, the 1case, the 1 stst body body segment becomes a segment becomes a copy of the 2copy of the 2 ndnd body segment. The body segment. The head wil l have an head wil l have an abnormal coding abnormal coding (gain(gain --ofof -- function function homeotic mutation); homeotic mutation); wil l produce wil l produce

posteriorizationposteriorization •• Genes that encode Genes that encode

transcription transcription factorsfactors

•• Encode a Encode a sequence of 60 sequence of 60 basic amino acids basic amino acids which form their which form their DNA binding DNA binding domain domain (homeodomain)(homeodomain)

•• Mostly concerned Mostly concerned with developmentwith development

•• Genes that are Genes that are

responsible for responsible for determining the determining the anteroposterior anteroposterior identity of bodidentity of body y levels.levels.

•• Activated at an Activated at an early stage in early stage in bodybody--plan plan formationformation

•• Maximally Maximally expressed around expressed around the phylotypic the phylotypic

stagestage •• Expressed in both Expressed in both

CNS and mesodermCNS and mesoderm •• Includes Includes

movements and movements and changes in cell changes in cell shapeshape

•• Achieved through Achieved through gastrulationgastrulation

•• CELL MOVEMENTCELL MOVEMENT oo Important in Important in

migration, migration, differential adhesion differential adhesion

or shape changeor shape change oo Evident in f ibroblasts Evident in f ibroblasts

with lamell ipodium, with lamell ipodium, embryo cells with embryo cells with f i lopodia, cells that f i lopodia, cells that use motor proteinsuse motor proteins

•• CELL ADHESIONCELL ADHESION oo An important An important

property of property of embryonic embryonic structures wherstructures where l ike e l ike cells tend to stick cells tend to stick together and sort together and sort out from cells of a out from cells of a

different kind.different kind. oo FUNCTIONS: FUNCTIONS:

hold together hold together components of sol id components of sol id t issuestissues

important for the important for the function of migratory function of migratory cellscells

important during important during embryonic embryonic development for the development for the process of process of morphmorphogenesisogenesis

•• CELL ADHESIONCELL ADHESION oo CaCa--mediated mediated

adhesionadhesion Calcium ions bind Calcium ions bind

glycoproteins glycoproteins protruding from protruding from the surface of two the surface of two adjacent cellsadjacent cells

Cadherin Cadherin –– the the glycoprotein glycoprotein involved in this involved in this type of adhesiontype of adhesion

1. E 1. E --cadherin cadherin (uvomorulin(uvomorulin or or LL --CAM) found in CAM) found in many types of many types of epithelial cells and epithelial cells and

cleaving mammalian cleaving mammalian embryosembryos

•• CELL ADHESIONCELL ADHESION oo CaCa--mediated mediated

adhesionadhesion 2. 2. NN--cadherincadherin found on cells of found on cells of the heart, lens, and the heart, lens, and nerves, and is also nerves, and is also expressed in the expressed in the mesodermal cells mesodermal cells of embryos during of embryos during gastrulgastrulation and ation and morphogenesis; morphogenesis;

expressed in the expressed in the neural cells and the neural cells and the mesenchymal cells mesenchymal cells l ike f ibroblasts; l ike f ibroblasts; associated with cell associated with cell moti l ity and moti l ity and invasion.invasion.

•• CELL ADHESIONCELL ADHESION oo CaCa--mediated mediated

adhesionadhesion 3. 3. PP--cadherincadherin found found on the placenta on the placenta and certain and certain epithelial epithelial cells; cells;

engaged in various engaged in various cellular activit ies cellular activit ies including moti l ity, including moti l ity, invasion, and invasion, and signaling of tumor signaling of tumor cells, in addition to cells, in addition to cell adhesion. cell adhesion.

In the absence of In the absence of calcium, exposed calcium, exposed cadherins are cadherins are subjected to subjected to proteolysis.proteolysis.

•• CELL ADHESIONCELL ADHESION oo Homophilic bindingHomophilic binding

BB inding between inding between l ike moleculesl ike molecules

Involves NInvolves N--CAM CAM (neural cell (neural cell adhesion adhesion molecule)molecule)

NN--CAM molecules CAM molecules of adjacent cells of adjacent cells bind directly with bind directly with one another in the one another in the absence of absence of calciumcalcium

Involved in a Involved in a number of number of morphogenetic morphogenetic

events during events during embryogenesisembryogenesis

•• CELL ADHESIONCELL ADHESION oo Heterophilic bindingHeterophilic binding

Lock and key Lock and key fashion between fashion between complementary complementary saccharidessaccharides

Occurs during Occurs during mammalian mammalian ferti l ization, when ferti l ization, when the head of a the head of a spermatozoon spermatozoon encounters the encounters the

membrane (zona membrane (zona pellucida) pellucida) surrounding the surrounding the egg.egg.

•• CELL ADHESIONCELL ADHESION

oo Tight junctionTight junction seal adjacent seal adjacent

epithelial cellsepithelial cells form fluidform fluid -- t ight t ight

seals between seals between cells l ike that in a cells l ike that in a Ziploc bag thus Ziploc bag thus forming an forming an

impermeable impermeable barrier to the barrier to the outside.outside.

Common among Common among epithelial cells epithelial cells that l ine the that l ine the stomach, intestine, stomach, intestine, and urinary and urinary bladder. bladder.

•• CELL ADHESIONCELL ADHESION oo Tight junctionTight junction

They prevent f luid They prevent f luid in a cavity from in a cavity from leaking into the leaking into the

body by passing body by passing between cells.between cells.

block movement block movement of integral of integral membrane membrane proteins between proteins between the apical and the apical and basolateral basolateral surfaces of the surfaces of the cell. cell.

•• CELL ADHESIONCELL ADHESION oo Adherens junctionAdherens junction

built from: built from: cadherins cadherins --

transmembrane transmembrane proteins whose proteins whose extracellular extracellular segments bind to segments bind to each other and each other and whose intracellular whose intracellular segments bind to segments bind to cateninscatenins

catenins catenins -- connected to connected to actin actin microfi laments of microfi laments of the cytoskeletonthe cytoskeleton

•• CELL ADHESIONCELL ADHESION oo Adherens junctionAdherens junction

provide strong provide strong mechanical mechanical attachments attachments between adjacent between adjacent cells by cells by connecting to the connecting to the microfi laments of microfi laments of the cytoskeleton.the cytoskeleton.

hold cardiac hold cardiac muscle cells t ightly muscle cells t ightly together as the together as the heart expands heart expands and contracts. and contracts.

•• CELL ADHESIONCELL ADHESION oo Adherens junctionAdherens junction

May anchor cells May anchor cells to extracellular to extracellular matrix.matrix.

May be May be responsible for responsible for contact inhibit ion. contact inhibit ion.

Loss of functioning Loss of functioning adherens junctions adherens junctions may also lead to may also lead to tumor metastasis. tumor metastasis.

•• CELL ADHESIONCELL ADHESION oo DesmosomesDesmosomes

Localized patches Localized patches that hold two cells that hold two cells t ightly together in t ightly together in small spots. small spots.

common in common in epithelia (e.g., the epithelia (e.g., the skin). skin).

serve as focal serve as focal points for the points for the attachment of attachment of intermediate intermediate fi lamentsfi laments

•• CONDENSATIONCONDENSATION •• INVAGINATIONINVAGINATION •• INVOLUTIONINVOLUTION •• CAVITATIONCAVITATION •• DELAMINATIONDELAMINATION •• CONVERGENT CONVERGENT

EXTENSIONEXTENSION •• PLANAR CELL PLANAR CELL

POLARITYPOLARITY

•• EPIBOLYEPIBOLY •• BRANCHING BRANCHING

MORPHOGENESISMORPHOGENESIS

•• Growth involves Growth involves

cell divis ioncell divis ion •• Apoptosis Apoptosis ––

programmed cell programmed cell

deathdeath •• Involves a Involves a

molecular pathway molecular pathway that culminates in that culminates in the activation of the activation of proteases cproteases called alled caspasescaspases

• May consist of short migrations by individual cells or massive dislocation of groups of cells (ex. Epithelial cells)

• Examples: migration of neural crest cells, spreading out of mesodermal cells, movement of epithelial cells

• process by which cells commit suicide; it is repeated in the embryo a number of times

• a form of programmed cell death

• Genetically determined.

• involves a series of biochemical events leading to variety of morphological

changes, like changes to the cell membrane such as loss of membrane asymmetry and attachment, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation.

•• FUNCTIONS:FUNCTIONS: 1. Cell Termination.

o Occur when a cell is damaged beyond repair, infected with a virus, or undergoing stress conditions such as starvation.

o DNA damage from ionizing radiation or toxic chemicals may induce apoptosis via the actions of the

tumour-suppressing gene p53. o The "decision" for apoptosis can

come from the cell itself, from the surrounding tissue, or from a cell that is part of the immune system.

•• FUNCTIONS:FUNCTIONS: 1. Cell Termination.

o In these cases apoptosis functions to remove the damaged cell, preventing it from sapping further nutrients from the organism, or to prevent the spread of viral infection.

o Apoptosis also plays a role in preventing cancer

•• FUNCTIONS:FUNCTIONS: 2. Homeostasis.

o To keep the number of cells relatively constant through cell death and division. Cells must be replaced when they become diseased or malfunctioning; but proliferation must be compensated by cell death. Required by living organisms to maintain their internal states within certain limits.

o Consequence of disrupting homeostasis: if cells are dividing faster than they

die, a tumor is developed. if cells are dividing slower than

they die, a cell loss disorder results.

•• FUNCTIONS:FUNCTIONS: 3. Development.

o Development of an organ or tissue is often preceded by the extensive division and differentiation of a particular cell, the resultant mass is then "pruned" into the correct form by apoptosis.

o During development, apoptosis is tightly regulated and different tissues use different signals for inducing apoptosis. Bone morphogenetic proteins (BMP) signaling is used to induce apoptosis in the interdigital tissue.

• Increase in mass • Embryos exhibit differential

growth (unproportional

increase in dimensions) • Major patterns in animals: 1. Determinate growth -

Body grows to a point that is characteristic of the species then ceases

2. Indeterminate growth - Growth continues throughout the lifespan but reduced rate