DEVELOPMEXT. 781 



the two layers forms the true amnion, while the outer or reflected layer, 

 sometimes termed the false amnion, coalesces with the inner surface of 

 the original vitelline membrane to form the subzonal membrane or 

 false chorion. This growth of the amniotic folds must of course be 

 clearly distinguished from the very similar process, already described, by 

 which at a much earlier stage the walls of the neural canal are formed. 



The cavity between the true amnion and the external surface of the 

 embryo becomes a closed space, termed the amniotic cavity (ac, fig. 473). 



At first, the amnion closely invests the embryo, but it becomes grad- 

 ually distended with fluid (liquor amnii), which, as pregnancy advances, 

 reaches a considerable quantity. 



This fluid consists of water containing small quantities of albumen 

 and urea. Its chief function during gestation appears to be the me- 

 chanical one of affording equal support to the embryo on all sides, and 

 of protecting it as far as possible from the effects of blows and other 

 injuries to the abdomen of the mother. 



The embryo up to the end of pregnancy is thus immersed in fluid, 

 which during parturition serves the important purpose of gradually and 

 evenly dilating the neck of the uterus to allow of the passage of the foetus : 

 when this is accomplished the amniotic sac bursts, and the waters escape. 



On referring to figs. 471, 472 and 473, it will be obvious that the 

 cavity outside the amnion, between it and the false amnion, is continu- 

 ous with the pleuro-peritoneal cavity at the umbilicus. This cavity is 

 not entirely obliterated even at birth, and contains a small quantity of 

 fluid, which is discharged during parturition either before, or at the 

 same time as the amniotic fluid. 



Allantois. Into the pleuro-peritoneal space the allantois sprouts 

 out, its formation commencing during the development of the amnion. 



Growing out from or near the hinder portion of the intestinal canal 

 (c, fig. 476), with which it communicates, the allantois is at first a solid 

 pear-shaped mass of splanchnopleure ; but becoming vesicular by the 

 projection into it of a hollow outgrowth of hypoblast, and very soon 

 simply membraneous and vascular, it insinuates itself between the amni- 

 otic folds, just described, and comes into close contact and union with 

 the outer of the two folds, which has itself, as before said, become one 

 with the external investing membrane of the egg. As it grows, the 

 allantois develops muscular tissue in its external wall and becomes ex- 

 ceedingly vascular; in birds (fig. 477) it envelops the whole embryo 

 taking up vessels, so to speak, to the outer investing membrane of the 

 egg, and lining the inner surface of the shell with a vascular membrane, 

 by these means affording an extensive surface in which the blood may 

 be aerated. In the human subject and in other mammalia, the vessels 

 carried out by the allantois are distributed only to a special part of the 



