COLUMBA, 



Zll 



day the edge of the epiblast has reached the equator and eventually 

 completes the enclosure by the sixteenth or seventeenth day. 



Fig. 271. — Section THROUGH a Chick's Egg 

 AT Various Stages. 



(After Duval.) 

 Segmentation Cavity. 



Hypoblast. 

 Blastopor 



Archenteron. 



Yolk-nuclei. 



Blastopore. Epiblasc. 



^ ' Hypoblast. 

 Mesoblast. 



Edge of 

 Blastoderm."! 



Primitive Groove 



Archenteron, 



Hypo- 



Yolk-nuclei, blast. 



In each case only the blastodermic pole is shown, the large mass of yolk being 

 cut off below. A, Section through the egg at blastula stage. B, Longitudinal 

 median section of the unincubated egg at the gastrula stage. C, Cross-section 

 through the blastopore of same stage. D, The same further forward. 



The crescentic groove, we already showed, was comparable to the 

 blastopore, and, after conversion into a longitudinal groove, it is known 

 as the primitive groove. The cells on either side of it are thickened 



