GASTRULA-STAGE 



563 



atioii-cavity (hi. avl) gradually disappears, llie edges of the lower'margin 

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

 plane, give rise to a vertical groove, the primilivc ,i;rciovc, as it is 

 called. 



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

 formed Ijy invagination, 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 arclientcron 



hd. 



pr.st 



jne.i 



I'lc. T45, — Two stages in the development of the bhistoderni of the cllielc, ;Lt aljunt 

 ttie twentieth and twenty-fourth iiour of incubation respecti%"ely ; diagrammatic. 

 ay. op. area opaca ; ar.pL. area pellucida; hil. head ; incd.f^r. medullary groove ; 

 itics. mesoderm, indicated ijy doited outline and deeper shade; pr.atii. pro- 

 amnion; pr. .,/. priiniti\-e streak and groove; /?-. r'. mesodermal segments or 

 protovertehra^. (From Marshall's EinbTyotogy^ in part after ]^)uval.) 



l)eing relatively very small and the ectoderm separated from the 

 endoderm by the yolk. 



The gastrula-stage is much less clearly distinguishable in the 

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

 the relatively enormous mass of unsegmented yolk is, as in the crayfisl!, 

 sufficient to nourish the embryo until it has' readied a st;ige closely 

 resembling the adult in almost every essential respect exce]it si/c. A 

 blastt^pore can sometimes be recognised in such cases, but in tlie 

 embryo of the common fowl it is only represented by -a priinilive i^yomr 

 (see above and Fig. 145 //■. st). The lilastoderm soon becomes difler- 



2 



