COLUMBA. 377 
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 CuHICcK’s Ecc 
AT VARIOUS STAGES. 
(After Duvat.) 
Segmentation Cavity. 
Hypoblast. Epiblast. | Archenteron. 
Blastopore. CPE 
0 Ons Onc} 
Eee? 8 Heke 
ER GH eIO ES 
~ Hypoblast. 
Mesoblast. Leg 
Edge of Soe Oh 
Blastoderm. Primitive Groove. Epiblast. 
eA rere os 
Archenteron. Mesoblast. 
8 Hy, 
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 
