448 OUTLINES OF CHORDATE DEVELOPMENT 



blood. The two regions are in actual connection only margin- 

 ally and through the fixation villi (Figs. 184, 185); later certain 

 septa afford additional connection (see below). From the 

 preceding description it is apparent that the human placenta is 

 at first of the diffuse type, later becoming discoid. 



The fully formed placenta (i.e., at nine months) is an oval 

 or circular disc of irregular outline, roughly 16-20 cm. in 

 diameter, and about 3 cm. in thickness toward the middle, 

 gradually becoming thinner toward the margin. The umbilical 

 cord transmitting the umbilical arteries and veins, is attached 

 eccentrically; the cord itself measures, at full term, about 

 50-60 cm. in length and has a diameter of something over 1 cm. 

 The villous surface of the placenta is marked out in irregular 

 areas by partitions or septa, extending upward from the decidua 

 basalis, some of which, when the placenta is in situ, attach to 

 the chorion and so divide the intra-placental or inter villous 

 cavity into chambers. These areas are known as the loculi, or 

 sometimes as cotyledons, and they must not be confused with 

 the groups of villi of the cotyledonary placenta of the Rumi- 

 nants. The villi extend from the surface of the septa as well as 

 from the general chorionic surface. 



Examination of Fig. 185, illustrating diagrammatically a 

 section through the placenta at its margin, will serve to make 

 clear the structure of the placenta. The embryonic surface of 

 the placenta, which is toward the top of the page, is seen to 

 be covered with the thin amnion; that is, the exoccelom has 

 been obliterated by the apposition of the amnion and chorion, 

 and the cavity of the embryonic vesicle is therefore the amnionic 

 cavity, filled with the amnionic fluid. 



The chorion presents first a connective tissue membrane 

 (close vertical ruling) beneath which are the vascular villi 

 (horizontal ruling) which in reality form a dense spongy mass. 

 The vessels extend through the complexly branched villi, 

 both types of which are shown. Toward the close of gestation 

 the fixation villi become very loosely attached to the decidua 

 basalis, in preparation for the separation of the placenta at 

 parturition. The villi are for the most part freely suspended 



