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128 t3lastoueres: 11 hr. 71 th Cleavage. Large-cell~l blastula (morula). Blastomeres occupy ~ of the yulk surface. There is no blas~oco~l. Stag09: 'small-c.elled Blastula (1'lorula): 14 hr. I.nG.ivid.ual cells r::..ay still Le distitlguished. Blastomeres are bulging upward frOffi tne rOll.ncied yolk mass, and. occupy i of the yolk su:ci"ace. Blastom.0re Id.,yel's become progr':::3sively thill~"ler to~iard. t(le periphery of the blastoderm. There is no olastocoel. Stage 10: Norula: - 24 Dr. (Fie:;. 2c). Individual cells fua./ 1..0 lo.n'eer be di stiLlguished. except under veri iligc. power wi tn the (;horioli l'':'::liloved.. 'l'he blastodt::rm has a gran- ular appe~rance: ufldersu.r.face i'lat ly1.(.i.e on flattened surface or yolk sphere. The blastocoel has not Yt::t appeared. huu of Cl~avage. Stage 11: Blastula (Epiboly of Blascoderm):day 2; 28 hI'; 1.8 mra dia. (Fig. 2d) ih8 blastoderm has begun to spread out over the yolk sphere, aHd. has thil.l11ed as a reaul t (blas\:,odi se) . 'fhe blastocoel is present. 'I'cw p.::;riblast is visible as a I:1m of celis alOli.g the periphery of the blascoJerm: this ""ill fOI'm the inner layer of the yolk sac; the out- er layer will La formed from epiblast derived from the blastoderm. 'l'he blastoderm no lo,u.ger bulges from the yol~. but caps the top t of t;he sphere. Stage 12: Early Gas~I'ula: J4hr. (Fig. 2e). 'l'he under-1.'im of the blastodisc ha.s thicb:e.cJ.ed to form the "rand-- ~~ul~i;,r: vI' lil.ir't;;i.lla.l ridge w!lich co;:nes to i-iave aL-l i.rUler layer termed the ger;>! ring. j;lostcrlorly, the ring is thickest and is recognized as th8 emLrJonic shield. Presumptive ~fidoJermal calls at ~he pos~- erior e~ge of t~e s~ield have tegun to involute chrough the blasto- po.c~ dUU siJl'ec:td beneath tile olast.oJ(;I'111. Cells of tile prechordal plcl't(::::t.1U. Hocochol'Q (whici1 cS~::..l.l.lish the embrYOtlic axis) have begun .;;lg.('.;.ll.i....g illwarJ over t,rw dors.:ll lip oC the blastopore. Presurupti ve ilh,.;..;iuJ.er;:::.al cells ilc:.:.ve also t;ulT.l.eJ. iIJ.lrJi:l.l.'1.! ..llld h~ve posl tioned them- , ' "-., --_~'l"'0'7~~'~-;_,~.-.,_,~' ~',-<~::-_~)~ """0-<;,"--:> ....,.,~