416 



EMBRYOLOGY OF THE CHICK. 



quite closed. In front the neural tube has become swollen 

 forming the first cerehral vesicle, and from each side of this 

 vesicle, by the rapid proliferation of the epiblastic cells, two 

 processes grow out — the optic vesicles. A second and third 

 vesicle then appear behind the first, and behind the third 

 vesicle two pits, the auditory pits, the future organs of hear- 

 ing. During the first half of the second day the heart (Ht) 

 appears in the head-fold, its origin being connected with that 

 cleavage of the mesoblast already referred to. It is formed in the 

 pleuro-peritoneal space shown in fig. 200, and situated just beneath 

 the developing fore-gut (al). At first the heart is flask-shaped, 



— 7ne$. 



Fio. 200. — Transverse Section through Posterior Part of the Head of 

 AN Embryo Chick of Thirty Hours. 



Bhj Hind-brain ; Cr, vagus nerve ; E}\ epiblast ; not, notoehord ; mes, mesoblastj 

 al, throat ; Ht, heart ; Hy, hypoblast. (After Foster and Balfour.) 



with a swelling at its anterior end and another behind, the future 

 auricles. The heart is formed entirely from mesoblast, on the 

 ventral side of the throat, by a curious process of hollowing out 

 of the splanchnic mesoblast : some of the original central cells 

 become blood - corpuscles. Concomitant with the appearance 

 of the heart is the vascular system, which consists of a number 

 of tubes hollowed out in the mesoblast. Blood-corpuscles are 

 rapidly formed in masses in the vascular area, called blood-islands. 

 By degrees the heart becomes S-shaped, the right-hand curve of 



