GASTRULA-STAGE 



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ation-cavity (bl. cal) gradually disappears, the edges of the;lower'margin 

 of the blastopore approach one another, and uniting in the median 

 plane, give rise to a vertical groove, the primitive groove, as it is 

 called. 



In the centrolecithal egg of the crayfish (Fig. 91) a gastrula-stage is 

 formed by invs^inalion, but as the centre of the oosperm is filled with 

 solid yolk in the place of a segmentation-cavity containing fluid, the 

 invagination only extends a short distance inwards, the archenteron 



/id 



phsl 



Fig. 145. — Two stages in the development of the ^ blastoderm of the chick, at about 

 the twentieth and twenty-fourth hour of incubation respectively ; diagrammatic. 

 ar. op. area opaca ; ar.pL. area pellucida ; hd. head ; incd.gr. medullary groove ; 

 mes. mesoderm, indicated by .dotted outline and deeper shade ; pr. am. pro- 

 amnion ; pr. St. primitive streak and groove ; pr. v. mesodermal segments or 

 protovertebree. (From Marshall's JEmbryology, in part after Duval.) 



being relatively very small and the ectoderm separated from the 

 endoderm by the yolk. 



The gastrula-stage is much less clearly distingtiishable in the 

 segmenting eggs of the dogfish and bird (pp. 454 and 547), in which 

 the relatively enormous mass of unsegmented yolk is, as in the crayfish, 

 sufficient to nourish the embryo until it has reached a stage closely 

 resembling the adult in almost every essential respect except size. A 

 blastopore can sometimes be recognised in such cases, but in the 

 embryo of the common fowl it is only represented by s. primitive groove 

 (see above and Fig. 145 pr. st). The blastoderm soon becomes differ- 



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