80 



THE SECOND DAY. 



[chap. 



An Embryo Chick of about Thirty-six Hours, viewed 

 from below as a transparent object. 



FB. the fore-brain or first cerebral vesicle, projecting from the 

 sides of which are seen the optic vesicles, op. A definite 

 head is now constituted, the backward limit of the somato- 

 pleure fold being indicated by the faint line S.O. Around 

 the head are seen the two limbs of the amniotic head-fold : 

 one, the true amnion a, closely enveloping the head, the 

 other, the false amnion a', at some distance from it. The 

 head is seen to project beyond the anterior limit of the 

 pellucid area. 



The splanchnopleure folds extend as far back as sp. Along its 

 diverging limbs are seen the conspicuous venous roots of 



