708 



THE F(ETAL MEMBRANES. 



is reduced to its simplest form, and consists in little more than the 

 implantation of comparatively simple and diffused chorionic villi in 

 minute recesses of the vascular lining membrane of the uterus ; in 

 others there is a greater or less degree of deeper interpenetration of 



Fifr. 5in. 



Fig. 516. — Surface and Profile Views 

 OP THE Ovum of the Eabbit at the 



TIME OF THE FORMATION OF THE 



Chorion (KoUiker after Bischoff ). 



A and B, an ovum of 3 lines in 

 diameter ; C, one of 4 lines. «, the 

 chorion, with commencing villi ; h, the 

 vesicular blastoderm ; c, the thickened 

 part forming the embryonic area ; d, the 

 increasing extent in which the blastoderm 

 was found to consist of two layers. 



the more highly developed and complex villi with a vascular structure 

 formed from the uterine lining membrane, and which, from its being 

 in whole or in part separated along with the ovum from the uterus 

 in certain animals at the period ol^ birth, has received the name of 

 decidua. 



Origin of the Chorion.— The manner in -^-hich the permanent chorion is fii'st 

 formed has not yet Ijeen fully ascertained. The deposit of an albuminous layer on 

 the external surface of the zona pellucida of the rabbit, which takes place in the 

 course of the descent of the ovum through the Fallopian tube, naturally led to the 

 supposition that the chorion might be derived from some external deposit or uterine 

 secretion of this natm-e ; Ijut the fact that a similar deposit from without has not 

 been obseiwed to occur in other animals, and that the albuminous coat in the rabbit 

 very soon thins away like the zona itself, and gives place to other structures, has 

 caused this view to he abandoned. Xor is it probable that the chorion proceeds 

 mainly, as held by some, from a development of the vitelline membrane. For 

 when the rapid expansion of the o\-um occm-s shortly after its arrival in the 

 cavity of the uterus, the zona pellucida becomes proportionally dilated, and is 

 reduced to an extreme degree of thinness, so that at this period it is liable to be 



