DEVELOPMENT OF THE EMBRYO 



451 



yolk, which is the mechanical reason for the disc-shaped blastoderm 

 being where it is and what it is in the bird's egg. That is, if the large 

 yolk of a bird's egg were removed and the blastoderm were allowed to 

 assume the spherical shape it would naturally take due to surface ten- 

 sion, there would be a decided similarity between the disc-shaped blasto- 

 derm and the ordinary morula stage of eggs with little yolk, such as in 

 amphioxus and in man. 



Not only does the great quantity of yolk make this change in the 

 morula stage, but it is evident that a large amount of yolk does not per- 

 mit a simple hollow sphere formation by any method of cell arrange- 

 ment. Nevertheless, the central cells do separate somewhat from the 

 yolk and form the slight segmentation cavity mentioned above. 



Imagining, now, that the yolk could be removed and the ends of the 

 blastoderm drawn together, we should have a true blastula form of the 

 simpler type. 



GASTRULATION 



It is essential that one remember that a large quantity of yolk will 

 make a considerable change in the process of gastrulation. The simpler 



Y*//C 



folk 



Fig. 260. 



Gastrulation in egg with different quantities of yolk. 1-5, Amphioxus (little 

 yolk) ; 6-8 Amphibian (moderate amount of yolk) ; 9-10, Birds (large amount of 

 yolk); blc, blastocoele ; future dorsal side; ect., ectoderm; end., entoderm ; ent. 

 and ach., archenteron ; blp., blastopore ; y.p., yolk plug. (After various authors.) 



