DEVELOPMENT. 779 



It passes in at the point where the notochord falls into the primitive 

 streak. The anterior part of the primitive streak becomes the tail 

 swelling, the posterior part atrophies, and the corresponding lateral 

 part of the blastoderm forms part of the body-wall of the embryo. 



The anterior part of the medullary canal having been completely 

 roofed in, the foremost portion undergoes dilatation, and a bulb, the 

 first or anterior cerebral vesicle, results. From either side of this 

 dilatation a process, the cavity of which is in communication with it, 

 is separated off, which is called the optic vesicle. 



Behind the first cerebral vesicle two other vesicles now arise, the 

 second or middle, and the third or posterior cerebral vesicle, and 

 at the posterior part of the head- two small pits, the auditory vesicles 

 or pits, are to be seen. The folding of the head, it should be recol- 

 lected, is the cause of the inclosure below the neural canal (fig. 469) of 

 a canal ending blindly, which has in front the splanchnopleure, and 

 which is just as long as the involution of that membrane. This canal 

 is the fore-gut. In the interior of the splanchnopleure fold below it 

 (as seen in fig. 469) in the pleuro -peritoneal cavity the heart is formed, 

 at the point where the splanchnopleure makes its turn forward. It 

 arises as a thickening of the mesoblast on either side as the two splanchno- 

 pleure folds diverge, and of a thickening of the mesoblast at the point 

 of divergence. So that at first the rudiment of the heart is like an 

 inverted V, which by the gradual coming together of the diverging 

 cords is converted into an inverted Y. 



The cylinders become hollowed out, and are thus converted into 

 tubes, which then coalesce. Layers are separated off toward the interior, 

 which become the epithelial lining, and the mass of the mesoblast sur- 

 rounding this, afterward form the muscle and serous covering, while at 

 first the rudimentary organ is attached to the gut by a mesoblastic mes- 

 entery, the mesocardium. 



FCETAL MEMBRANES. 



Umbilical Vesicle (Yolk-sac). The splanchnopleure, lined by hy- 

 poblast, forms the yolk-sac in reptiles, birds, and mammals; but in 

 amphibia and fishes, since there is neither amnion nor allantois, the wall 

 of the yolk-sac consists of all three layers of the blastoderm, inclosed, of 

 course, by the original vitelline membrane. 



The body of the embryo becomes in great measure detached from 

 the yolk-sac or umbilical vesicle, which contains, however, the greater 

 part of the substance of the yolk, and furnishes a source whence nutri- 

 ment is derived for the embryo. This nutriment is absorbed by the 

 numerous vessels (omphalo-mesenteric) which ramify in the walls of the 

 yolk-sac, forming what in birds is termed the area vasculosa. In 



