1 82 H. H. NEWMAN AND J. THOS. PATTERSON. 



An examination of the external villous layer of the chorion shows 

 that placentation is unique in that areas of villi, although appar- 

 ently forming a complete zone about the equatorial region of the 

 uterine wall, are in reality arranged in four closely approximated 

 ovoid areas, two large and lateral in position, and two smaller, 

 one dorsal and the other ventral. The proximal (/. e., the pos- 

 terior or vaginal) and distal ends of the chorionic vesicle are 

 practically free of villi, except at each pole, where the presence 

 of a small tuft of villi causes the chorion to adhere very firmly to 

 the uterine wall (see Fig. i). The villous zone, composed as it 

 is of four ovoid areas, extends into the polar regions in the form 

 of four scallops at each end, and on the amniotic side of the four 

 scallops are situated the points of attachment of the four umbilical 

 cords (see Fig. 2). 



In the advanced stages of gestation the chorionic vesicle is 

 shaped very much like a football, and the four lines on the inside 

 wall, along which the adjacent amnia meet, correspond to the 

 seams of the ball (see Fig. i). In the young stages, however, 

 the chorion is in the shape of an octaedron with rounded edges, 

 and flattened dorso-ventrally, and has its entire surface covered 

 with villi (see Fig. 3). From this we may conclude that the 

 definitive condition of the placenta has been attained by a locali- 

 zation of the villi into ovoid areas, each of which is to be looked 

 upon as the vascular center for an umbilicus. 



The points of attachment of the umbilical cords lie close to 

 the amniotic partitions, and when the chorion is viewed from the 

 distal end the cord in each case is seen to be situated just to the 

 right of the partition (see Fig. 2). In embryos about one fourth 

 grown the partition between any two contiguous embryos may 

 be easily separated into its component parts, but in older stages 

 the fusion of the membranes is so complete that separation is 

 impossible. In late stages, therefore, the chorionic cavity is 

 completely divided longitudinally into four separate compart- 

 ments, each of which contains an embryo. Although the por- 

 tions of the chorionic wall that lie between the successive amni- 

 otic partitions are in the form of quadrants, yet they do not 

 coincide with the ovoid areas ; for the partitions do not meet the 

 chorionic wall in lines corresponding to the divisions of the ovoid 



