THE DEVELOPMENT OF THE EMBRYO ITSELF. 101 



the embryo itself the circulation of the yelk-sac is established, 

 the allantois originates, and the amnion makes rapid progress. 



It may be noticed that the cerebral vesicles, the optic vesi- 

 cles, and the auditory pit are all derived from the epiblastic 

 accumulations which occur in the anterior extremity of the 

 embryo ; and their early appearance is prophetic of their physi- 

 ological importance. 



The heart, too, so essential for the nutrition of the embryo, 

 by distributing a constant blood-stream, is early formed, and 

 becomes functionally active. It arises beneath the hind-end of 

 the fore-gut, at the point of divergence of the folds of the 



B _. 



Fig. 104. — Diagram representing under surface of an embryo rabbit of nine days 

 tliree hours old^ illustrating development of the heart (after Allen Thomson). 



and 

 . A, 

 view of the entire embryo; B, an enlarged outline of the heart of A; C, later stage 

 of the development of B; hh, ununited heart; oo, aortee; m, vitillme veins. 



splanchnopleure, and so lies within the pleuro-peritoneal cav- 

 ity, and is derived from the mesoblast. At the beginning the 

 heart consists of two solid columns ununited in front at first ; 

 later, these fuse, in part, so that they have been compared 

 with an inverted Y, in which the heart itself would correspond 

 to the lower stem of the letter (j) ^nd the great veins (vitel- 

 line) to its main limbs. The solid cords of mesoblast become 



