202 EMBRY0L0GICAL TYPES 



is the dorsal lip of the blastopore, and the primitive streak 

 represents the lateral lips of the blastopore, fused together 

 along their whole length. All that is left of the aperture of 

 the blastopore is the primitive pit and the primitive groove. 

 This is the condition of the blastopore of the frog after its 



.~^V:'^^^ ap. 



n \ '-''' :V-'^ V X^V/ • ■'■'■W-S 



" '^:!4k-!nL'-SM;:.v1|-£: :•: •/ -ferMI 



}■* V.& III £**« 



T>- •-; .':y-4l ' If -' ..«fJ"* > 



Fig. 90. — Gallus : view of the blastoderm of a hen's egg after 20 hours' 

 incubation, as seen by transmitted light. (After Jenkinson.) 



ao, area opaca ; ap, area pellucida ; av, area vasculosa ; hf, head-fold ; 

 n, notochord ; nf, neural fold ; pa, proamnion ; ps, primitive streak. 



aperture has become slit-like and closed by the apposition of 

 its lateral lips. The chick therefore starts straight away from 

 the condition reached in the frog at the close of gastrulation. 

 The primitive streak, like the lip of a blastopore which it 

 is, is a region of cell-proliferation, and it gives rise to mesoderm- 



