GASTRULA-STAGE 



581 



being 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 (pp. 470 and 573), 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 



FIG. 156. Oosperm of rabbit 70-90 hours after impregnation. 



fo/. cavity of blastodermic vesicle (yolk-sac) ; ep. outer layer of cells (trophoblast) ; 

 hy, inner mass of cells of the embryonic area ; Zp. albuminous envelope. (From 

 Balfour's Embryology, after E. van Beneden.) 



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

 (see p. 580 and Fig. 155, pr. st). The blastoderm soon becomes differ- 

 entiated into an outer ectoderm and an inner, lower layer of cells 

 (compare Fig. 128), between which and the yolk the enteric cavity is 

 formed : a segmentation-cavity is hardly recognisable. As the embryo 

 develops, it becomes folded off from the yolk, which forms & yolk-sac on 

 its ventral side (Figs. 129 and 165). 



The minute egg of the rabbit and of most other Mammals, although 

 alecithal and undergoing a holoblastic segmentation, has presumably 



