A VES. 



143 



Fig. 109. An embryo chick of about thirty- 

 six HOURS VIEWED EROM 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 en- 

 veloping 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 fold extends as far back 

 as 8}). Along its diverging limbs are seen the 

 conspicuous venous roots of the vitelline veins, 

 uniting to form the heart li, already^ established 

 by the coalescence of two lateral halves which, 

 continuing forward as the bulbus arteriosus h.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; inc. posterior part of notocherd; e. parietal 

 mesoblast ; pi. outline of area pellucida ; pv. 

 primitive streak. 



*•« rh 



of the embryo. 

 Up to the third 

 day it is placed 

 symmetrical- 

 ly, on the yolk, 

 with its ventral 

 face downwards. 

 During this day 

 it turns so as 

 partially to lie 

 on its left side. 

 This rotation af- 

 fects first the 

 head (fig. Ill), 

 but in the course 

 of the fourth day 

 gradually ex- 

 tends to the rest 

 of the body (fig. 

 118). Coinci- 

 dently with this 

 change in posi- 

 tion the whole 

 embryo undergoes 



Fig. 110. Transverse section through the posterior 



PART OF the head OF AN EMBRYO CHICK OF THIRTY HOURS. 



lib. hind-brain; vg. vagus nerve; ep. epiblast; ch. noto- 

 chord ; x. thickening of hypoblast (possibly a rudiment of the 

 subnotochordal rod); al. throat; ht. heart; pp. body-cavity; 

 so. somatic mesoblast; sf. splanchnic mesoblast; liy. hypo- 

 blast. 

 a ventral and somewhat spiral flexure. 



