FORMATION OF THE CHORION. 609 



occupied by them. It is then termed the chorion, and thus becomes 

 the sole external investing membrane of the egg. 



We find, therefore, that the chorion, that is, the external coat or 

 investment of the egg, is formed successively by three distinct 

 membranes, as follows: first, the original vitelline membrane; 

 secondly, the outer lamina of the amniotic fold ; and, thirdly, the 

 allantois ; the last predominating over the two former by the rapidity 

 of its growth, and absorbing them into its substance, so that they 

 become finally completely incorporated with its texture. 



It is easy to see, also, how, in consequence of the above process, 

 the body of the foetus, in the human egg, becomes inclosed in two 

 distinct membranes, viz., the amnion, which is internal and conti- 

 nuous with the foetal integument, and the chorion, which is external 

 and supplied with vessels from the cavity of the abdomen. The 

 umbilical vesicle is, of course, situated between the two ; and the 

 rest of the space between the chorion and the amnion is occupied 

 by a semi-fluid gelatinous material, somewhat similar in appearance 

 to that of the vitreous body of the eye. 



The obliteration of the cavity of the allantois takes place very 

 early in the human subject, and, in fact, keeps pace almost entirely 

 with the progress of its growth ; so that this organ never presents, 

 in the human egg, the appearance of a hollow sac, filled with 

 fluid, but rather that of a flattened vascular membrane, enveloping 

 the body of the foetus, and forming the external membrane of the 

 egg. Notwithstanding this difference, however, the chorion of the 

 human subject, in respect to its mode of formation, is the same 

 thing with the allantois of the lower animals ; its chief peculiarity 

 consisting in the fact that its opposite surfaces are adherent to each 

 other, instead of remaining separate and inclosing a cavity filled 

 with fluid. 



The next peculiarity of the human chorion is, that it becomes 

 shaggy. Even while the egg is still very small, and has but recently 

 found its way into the uterine cavity, its exterior is already seen 

 to be covered with little transparent prominences, like so many 

 villi (Fig. 213), which increase the extent of its surface, and assist 

 in the absorption of fluids from without. The villi are at this time 

 quite simple in form, and altogether homogeneous in structure. 



As the egg increases in size, the villi rapidly elongate, and be- 

 come divided and ramified by the repeated budding and sprouting 

 of lateral offshoots from every part. After this process of growth 

 39 



