Early CleavageAfter fertilization, the frog zygote proceeds through radial, holoblastic cleavage. Because of the large mass ofyolk in the vegetal pole, the resulting blastomeres are of unequal sizes yeilding micromeres (1) at the animalpole and macromeres (2) at the vegetal pole with evident cleavage furrows (3) between them. Each one ofthese resulting cells contain a nucleus (4), which are often missed in the thin, cross sectionalcuts. At thecenter of the blastomeres one can see a cavity beginning to form called the blastocoel (5). Its obvious thatfertilization has occured because cleavage is occuring !duh"#, but also because the fertilization envolope (6)is visibly raised from the surface of the dividing cells. Along the periphery !especially at the animal pole# youcan see the layer of cortical pigments (). $his protects the eggs and early stages of development from directsunlight. $he yolk itself is composed of golgi%derived yol! platelets (").#lastula&ometimes referred to as 'late cleavage,' the blastula is a hollow ball of cells. $he cells at this stage continueto undergo rapid cell division (mitosis$1) creating a hollow space within the blastula called the blastocoel (2)%$his transient structure which will soon disappear during gastulation performs the short but important task ofpreventing cell%cell communication between the ectoderm at the animal pole and the endoderm below in thevegetal pole. $he point of sperm entrance will determine the anterior (&) and posterior (') a(is in the embryo.)ecanestimatethequadrant of spermentrancebasedonthecortical layer(3)%$hecounterclockwiserotation seen by the lower left margin of corticallayer(4)indicates that sperm entrance occured in the leftquadrant of the animal pole !the corticallayer rotates toward the point of sperm entrance#. Invagination willbegin opposite of sperm entrance (5)%Early (astrulaAse(pected, gastrulationbeginswiththeinvaginationof theblastopore(1)oppositeof spermentrance.$hroughout gastrulation, the ectoderm will proliferate causing the surface of the blastula to slide towards theblastopore (epiboly$2)and in (involution$3)to form the gut precursor orarc)enteron(4)%At this stage thearchenteron is little morethat a slit and the blastocoel (5)is stillvery prominent* however, as gastrulationproceeds, thearchenteronwill enlargeandtheblastocoel will bedisplacedtothesideandeventuallydisappear...its +ob is complete. Because frogs are deuterostomes, like other vertebrates, the blastopore markstheposterior (')regionwheretheproctodeum,anuswill form, withthestomodeumforminglater intheanterior (&)region. $he *orsal lip (6)is clearly visible, at the beginning of gastrulation, while the ventral lipwill form later. +ate (astrula$hepurposeof gastrulationistheformationof thethreeprimarygermlayers- endoderm, mesoderm, andectoderm. .uring late gastrulation all three of these are clearly visible with ecto*erm (1) surrounding the outerportionof theembryo,en*o*erm(2)surroundingthegut !archenteron# andcomposingtheyolk, andmeso*erm (3)inbetween. /otice that the blastocoel (4)has been pushed off to one side by the enlargingarc)enteron (5) and will not be seen again after this. Both the *orsal lip (6) and blastopore () are still visiblewith the addition of the ventral lip ("). $he yol! plug (,)is the endodermalmass between the dorsalandventral lips.0p to this point each of the other early development slides have been sagittal sections with anterior andposterior ends visible. 1ook at the neurula...where is anterior and posterior2 3ou dont see them because this isa cross section% $he neurula will not make sense to you if you do not understand this"-eurulaAftergastrulationthingsstart tohappenfast withneurulation, theformationof theneural tube. $hiscrosssection shows an embryo in the middle of neurulation with the neural fol*s (1) rising to form the neural tube.4n either side you can see the neural crest cells (2)beginning migration. Also present at this stage are allthree germ layers-ecto*erm(3). en*o*erm (4)and meso*erm%$he mesoderm can be subdivided into 5regions, 6 of which areshown here-lateralplate meso*erm (&%/%&%ventral meso*erm $ 5).notoc)or*(&%/%&% c)or*ameso*erm$ 6).andpara0ial meso*erm(&%/%&% *orsal meso*erm$ )%$heother twodivisions, head mesenchyme and intermediate mesoderm !A.7.A. middle mesoderm# are not clearly visible./ot to be confuse with the now absent blastocoel, the arc)enteron (") is very prominent.