746 DEVELOPMENT OF THE IMPREGNATED EGG. 



peritoneum in the adult. This space afterward enlarges somewhat and 

 becomes the amniotic 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 external 

 membrane, or chorion (Fig. 259;, is occupied by an amorphous gelati- 

 nous material, in which the umbilical vesicle and its stem lie imbedded. 

 Subsequent!}^ 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. 260) ; so that a considerable 

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

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

 this increased rate of growth until about the beginning of the fifth 

 month, when it comes in contact with the inner surface of the chorion ; 

 the gelatinous material previously intervening between them having 

 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 corresponding envelope in 

 the lower animals, by a protrusion or outgrowth from the posterior por- 

 tion of the primitive alimentary canal, which, insinuating itself between 

 the two laminae of the amniotic fold, spreads gradually over and around 

 the inner lamina or amnion proper, so as to occupy finally a position out- 

 side of it. It there meets with the two thin layers which have preceded 

 it in this situation, namely the outer lamina of the amniotic fold, and 

 the original vitelline membrane of the egg. But these two layers, ceas- 

 ing to grow, while the new structures and the whole egg are rapidly 

 enlarging, disappear as distinct membranes, and their place is taken by 

 the chorion, which thus becomes, alone, the external investment of the 

 egg- 



So far, the history of development of the chorion is the same with 

 that of the allantois. But the peculiarity which distinguishes it is that, 

 in expanding over and around the other parts, it does not present the 

 form of a double sac containing fluid, but of a single vascular sheet or 

 membrane, like that of the skin. It is on this account that in the 

 human species it is called the chorion, while in the lower animals it 

 retains the name of allantois. 



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

 a hollow sac or canal with a blind extremity, the cavity of which is a 

 continuation of that of the intestine. But this cavity does not extend 

 i\t any time for more than a short distance outside the body of the 

 embryo. Beyond this point it becomes obliterated, its membranous 

 walls remaining in contact with and adherent to each other, forming a 

 solid membrane, as above described. Inside the body of the embryo, 

 on the other hand, it retains the form of a membranous sac ; and this 



