440 



EMBltYOLOGY OF THE CHICK. 



quite closed. In front the neural tube has become swollen, 

 forming the first rerehval rudde, and from each side of this 

 vesicle, b}- the rapid proliferation of the epiblastic cells, two 

 processes grow out — the nidic raddvi. 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. 232, and situated just beneath 

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



■yn.es. 



Kx 



>>ICS 



I'm. 232- 



TkaNSVKBSE SKCTION THROnGH POSTERIOR PaRT Of THE HkA.D i 



AN Embryo Chick of Thirty Hours. 



///), Hind-brain; Cr, vagus nerve; Ep, epiblast ; not, notochord ; »(cs- mesoblast- 

 a!, tliroat; y», heart; //i/, liypoblast. (After Foster and Balfour.) ' 



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

 aui-iclf'S. The heart is formed entirely from mesoblast, on the 

 ventral side of the throat, by a curious process of hollo\\'in<T 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 



