REPRODUCTION 



79 



splanchnopleure and somatopleure with coelom between them. The 

 embryo is put to the necessity of building not only its enteron but its 

 body-wall around its prospective food. 



ectodsrm o( neural plat 

 ctodcrm of blastoderm 



fore-gut 



illantoic bud 

 yolk- stalk 



Fig. 72. — Diagrams representing median longitudinal sections of chick embryos 

 after incubation for approximately one day, A ; two days, B; three days, C; four days, D. 

 The four stages show progressive differentiation of the regions of the enteron and pro- 

 gressive constriction between the yolk-sac and the shaping body of the embryo. (From 

 Patten, "Embryology of the Chick.") 



In course of development the yolk is assimilated and utilized in the 

 building of new protoplasm. It therefore steadily decreases in bulk 

 both relatively and absolutely. As the body of the embryo begins to 



