REPRODUCTION 



III 



cells in the wall of the enteron is increased in diameter thirty or more 

 times, meanwhile the layers of the splanchnopleure and somatopleure 



ectoderm of neural plate 

 ectoderm of blastoderm 



yolk 



primitive pit 



primitive streak 





open neural groove 

 primitive pit 



Rathke's pocket 



allantoic bud 

 yolk- stalk 



Fig. 82. — Diagrams representing median longitudinal sections of chick embryos 

 after incubation for approximately one day, .4 ; 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.) 



Stretching to accomodate themselves to the swollen enteron. The result 



would be a section essentially like the corresponding section of some bird. 



Yolk is food. In an animal having an enteron, the appropriate place 



for food is in the enteron. In an amphibian embryo the relatively small 



