646 REPRODUCTION. 



becomes the anmiotic cavity, containing a little colorless, transparent, 

 serous fluid, the amniotic fluid. But throughout the earlier periods of 

 development the cavity of the amnion is small, as compared with that 

 of the entire egg ; and the space between the amnion and the exter- 

 nal membrane, or chorion (Fig. 198), is occupied by an amorphous 

 gelatinous material, in which the umbilical vesicle and its stem lie 

 imbedded. 



Subsequently the amnion enlarges more rapidly, in comparison with 

 the remaining parts of the egg, and thus encroaches upon the layer of 

 gelatinous material by which it is surrounded. At the same time the 

 amniotic fluid increases in quantity (Fig. 199) ; so that a considerable 

 space is left around the embryo, in which it is supported by the uniform 

 pressure of the amniotic fluid. The amnion continues to enlarge at 

 this increased rate until about the beginning of the fifth month, when 

 it comes in contact with the inner surface of the chorion ; the inter- 

 vening gelatinous material having by that time disappeared, or being 

 reduced to a nearly imperceptible layer. 



Chorion. 



The chorion, in the human species, is the external enveloping mem- 

 brane of the embryo. It originates, like the allantois in the lower ani- 

 mals, by an outgrowth from the posterior portion of the alimentary 

 canal, which, insinuating itself between the laminee of the amniotic 

 folds, spreads over and around the amnion, so as to occupy finally a 

 position outside of it. It there meets with the two thin layers which 

 have preceded it, namely, the outer lamina of the amniotic fold, and 

 the original vitelline membrane. But these layers, ceasing to grow, 

 while the new structures of the egg are rapidly enlarging, disappear 

 as distinct membranes, and are replaced by the chorion, which thus 

 becomes the external investment of the egg. 



So far, the development of the chorion is similar to that of the allan- 

 tois. But its distinguishing peculiarity is that, while extending over 

 and around the other parts, it does not present the form of a sac con- 

 taining fluid, but that of a vascular sheet or membrane, like the skin. 

 On this account it is called the chorion, while in the lower animals it 

 retains the name of allantois. 



Nevertheless, at its commencement, the chorion is, like the allantois, 

 a hollow membranous sac, the cavity of which is continuous with that 

 of the intestine. But this cavity extends only a short distance outside 

 the body of the embryo. Beyond this point its walls remain in contact 

 with each other, forming a single membrane. Inside the body of the 

 embryo, on the other hand, it retains the sac-like form ; and this por- 

 tion afterward becomes the urinary bladder. The rounded cord or 

 "urachus," which, in the adult, runs from the superior fundus of the 

 bladder to the umbilicus, is the vestige of the obliterated canal of the 

 primitive chorion. 



The next peculiarity of the chorion is, that it becomes shaggy. Even 



