96 ANIMAL PHYSIOLOGY. 
formed, but its curvature becomes more marked and rudiments 
of auricles arise; while outside the embryo itself the circula- 
tion 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 
Fic. 108.—Diagram representing under surface of an embryo rabbit of nine days and three 
hours old, illustrating development of the heart (after Allen Thomson). 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’; aa, aorte ; vv, vitelline veins. 
becomes functionally active. It arises beneath the hind-end of 
the fore-gut, at the point of divergence of the folds of the 
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 (4) and the great veins (vitelline) to its 
main limbs. The solid cords of mesoblast become hollow prior 
to their coalescence, when the two tubes become one. 
