DEVELOPMENT. 669 



foremost portion undergoes dilatation, and a bulb, or first cerebral 

 vesicle results. From either side of this dilatation a process, the cavity 

 of which is in communication with it, is separated off; these processes 

 are the optic vesicles. Behind the first cerebral vesicle two other vesicles 

 now arise, and at the posterior part of the head two small pits, the au- 

 ditory pits, are to be seen. The folding of the head, it should be rec- 

 ollected, is the cause of the inclosure below the neural canal (Fig. 451) 

 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. 451) in the pleuro-peritoneal cavity the heart is formed, 

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

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

 nopleure 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 in- 

 verted V, which by the gradual coming together of the diverging cords is 

 converted into an inverted Y. 



The cylinders become hollowed but, and are thus converted into tubes, 

 which then coalesce. Layers are separated off towards the interior, 

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

 rounding this afterwards forms the muscle and serous covering, whilst at 

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

 tery, the mesocardium. 



FCETAL MEMBRANES. 



Umbilical Vesicle or Yelk-sac. The splanchnopleure, lined by 

 hypoblast, forms the yelk-sac in Reptiles, Birds, and Mammals; but in 

 Amphibia and Fishes, since there is neither amnion nor allantois, the 

 wall of the yelk-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 

 yelk sac or umbilical vesicle, which contains, however, the greater part 

 of the substance of the yelk, and furnishes a source whence nutriment 

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

 vessels (omphalo-mesenteric) which ramify in the walls of the yelk-sac, 

 forming what in birds is termed the area vasculosa. In Birds, the 

 contents of the yelk-sac afford nourishment until the end of incubation, 

 and the omphalo-mesenteric vessels are developed to a corresponding 

 degree; but in Mammalia the office of the umbilical vesicle ceases at a 

 very early period, the quantity of the yelk is small, and the embryo soon 

 becomes independent of it by the connections it forms with the parent. 

 Moreover, in Birds, as the sac is emptied, it is gradually drawn into the 

 abdomen through the umbilical opening, which then closes over it: but 

 in Mammalia it always remains on the outside; and as it is emptied it 



