GENERATION AND DEVELOPMENT. 263 



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, some- 

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

 original vitelline membrane to form the 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. 



Amniotic Cavity. The cavity between the true amnion and the ex- 

 ternal surface of the embryo becomes a closed space, termed the amniotic 

 cavity (ac, Fig. 417). 



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 mechani- 

 cal one of affording equal support to the embryo on all sides, and of pro- 

 tecting 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 the diagrams (Fig. 417), 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 be- 

 fore, 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. 420), with which it communicates, the allantois is at first a solid 

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

 jection into it of a hollow outgrowth of hypoblast, and very soon simply 

 membranous and vascular, it insinuates itself between the amniotic 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 ex- 

 ternal investing membrane of the egg. As it grows, the allantois devel- 

 opes muscular tissue in its external wall and becomes exceedingly vascu- 

 lar; in birds (Fig. 421 ) it envelopes 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 



