172 



DEVELOPMENT DURING THE SECOND DAY. 



FIG. 109. 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 con- 

 stituted, the backward limit of the somatopleure 

 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 splanchriopleure fold extends as far back 

 as sp. Along its diverging limbs are seen the 

 conspicuous venous roots of the vitelline veins, 

 uniting to form the heart h, already established 

 by the coalescence of two lateral halves which, 

 continuing forward as the bulbus arteriosus b.a, 

 is lost in the substance of the head just in front 

 of the somatopleure fold. 



HB. hind-brain; MB. mid-brain; p.v. and 

 v.pl. mesoblastic somites; ch. front end of noto- 

 chord; me. posterior part of notochord; ^. parietal 

 mesoblast; //. outline of area pellucida; pv. 

 primitive streak. 



Ay 



j 



FIG. 1 10. TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE HEAD 



OF AN EMBRYO CHICK OF THIRTY HOURS. 



hb. hind-brain ; vg. vagus nerve ; ep. epiblast ; ch. notochord ; x. thickening of 

 hypoblast (possibly a rudiment of the subnotochordal rod); al. throat; hi. heart; 

 pp. body cavity ; so. somatic mesoblast ; sf. splanchnic mesoblast ; hy. hypoblast. 



