DEVELOPMENT. 671 



towards one point over its dorsal surface (Fig. 453). The growing up 

 of these folds from all sides and their convergence towards one point 

 very closely resembles the folding inwards of the visceral plates already 

 described, and hence, by some, the point at which the amniotic folds 

 meet over the back has been termed the amniotic umbilicus. 



The folds not only come into contact but coalesce. The inner of 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 the walls of the neural canal are formed at a much earlier 

 stage. 



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

 embryo becomes a closed space, termed the amniotic cavity (ac, Fig. 

 453). 



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

 gradually distended with fluid (liquor amnii), which, as pregnancy ad- 

 vances, 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 mechan- 

 ical 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 inju- 

 ries 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 foe- 

 tus: when this is accomplished the amniotic sac bursts, and the ' ( waters " 

 escape. 



On referring to the diagrams (Fig. 453), 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 ("false waters "), 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. 456), 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 out-growth of hypoblast, and very soon 

 simply membranous 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 



