ESTABLISHMENT OF THE ENTODERM 21 



detachment of its central cells from the underlying yolk while 

 the peripheral cells remain attached. The space thus estab- 

 lished between the blastoderm and the yolk is termed the seg- 

 mentation cavity (blastoccele). The marginal area of the 

 blastoderm in which the cells remain undetached from the yolk 

 and closely adherent to it, is called the zone of junction. With 

 the establishment of the blastoccele the embryo is said to have 

 progressed from the morula to the blastula stage. 



Figure 7, D, shows the conditions seen on sectioning the 

 blastula of a bird. Only the blastoderm and the immediately 

 underlying yolk are included in the diagram. At this mag- 

 nification the complete yolk must be imagined as about three 

 feet in diameter. The structure of the bird embryo in these 

 stages may be brought in line with the morula and blastula 

 stages of forms having little yolk if the full significance of the 

 great yolk mass is appreciated. Instead of being free to aggre- 

 gate first into a solid sphere of cells (morula) and then into a 

 hollow sphere of cells (blastula), as takes place in forms with K 

 little yolk, the blastomeres in the bird embryo are forced 

 to grow on the surface of a large yolk sphere. Under 

 such mechanical conditions the blastomeres are forced to be- 

 come arranged in a disc-shaped mass on the surface of the yolk. 

 If one imagines the yolk of the bird morula removed, and the 

 disc of cells left free to assume the spherical shape dictated by 

 surface tension its comparability with the morula in a form 

 having little yolk becomes apparent. 



The process of blastulation also is modified by the presence 

 of a large amount of yolk. There can be no simple hollow 

 sphere formation by rearrangement of the cells if the great 

 bulk of the morula is inert yolk. But the cells of the central 

 region of the blastoderm are nevertheless separated from the 

 yolk to form a small blastoccele. The yolk constitutes the 

 floor of the blastoccele and at the same time by reason of its- 

 great mass nearly obliterates it. If we imagine the yolk 

 removed from the blastula and the edges of the blastoderm 

 pulled together the chick blastula approaches the form of the 

 blastula in embryos with little yolk. 



The Effect of Yolk on Gastrulation. The process of gastrula- 

 tion begins as soon as blastulation is accomplished. Gastrula- 

 tion as it occurs in birds is not difficult to understand if one 



