DEVELOPMENT OF THE FECUNDATED EGG. 505 



14). In fact, the allantoi's, primarily vesicular, spreads into 

 a membrane which is filled with villi, which latter penetrate 

 into the villi of the second chorion. These villi of the 

 allantois are vascular, and, uniting with the second chorion 

 they form an enveloping membrane of the ovum, which defi- 

 nitely replaces the second chorion (Fig. 143, 15), and differs 

 from the latter by the fact that its membrane is vascular and 

 consequently capable of directly seeking by means of a 

 regular circulation (second circulation) the nutritive elements 

 supplied by the mother, and strained through the decidua y 

 whose formation we have previously studied (see Fig. 136). 

 This is what authors call the third or vascular chorion^ or 

 that membrane formed by the allantois, which afterwards 

 becomes the most external of the proper envelopes of the 

 egg, and which forms a covering for the remainder of the 

 second chorion (Fig. 142, 15). 



But these formations from the allantois last only for a 

 short time, especially in the human foetus. We have already 

 learned that those portions of the allantois which are nearest 

 the fcetus form successively the bladder and urachus (see p. 

 460) ; the other portion, from which the third chorion is 

 formed (Fig. 142, 15), is provided with vessels only on the 

 parts which correspond to the metnbrana decidua serotina 

 (see p. 497) ; everywhere else the vascular loops of the 

 villi undergo atrophy, and become the seat of no^ further 

 transformations at these points until the birth of the fcetus 

 (Fig. 143). 



The envelopes of the perfected ovum are everywhere the 

 same except at the placental attachments. Proceeding from 

 the outside towards the interior are (Fig. 143) : first, the 

 decidua, or rather deciduse (see p. 497), since, by the 

 acquired developments of the egg, the decidua reflexa is 

 in contact with the maternal decidua, or decidua vera (c, 

 Fig. 136), and since these two membranes are not distinct 

 from each other; however, they may be separated by a 

 dissection, and often a certain amount of fluid may be found 

 between the membranes (hydroperion of Velpeau) (21 and 

 23, Fig. 143) ; next comes the chorion (fusion of second and 

 third chorions, 19, Fig. 144), the cells and villi of which, after 

 the disappearance of the vessels, are united and fused in such 

 a manner as to form a homogeneous membrane, which is 

 more or less granular and interspersed with nuclei (Robin) ; 

 third, a layer of irregular cells > which are the remains of the 

 allantoid body, is formed below the chorion ; these have a stel- 



