128 



TEXT-BOOK OF EMBRYOLOGY. 



standpoint, as the finding of them in curettings or in a uterine discharge may 

 be accepted as proof of pregnancy. 



During the early months of pregnancy — first four months — the decidua 

 basalis has essentially the same structure as the decidua parietalis. Its surface 

 epithelium disappears very early, perhaps even before the attachment of the 

 ovum. The glandular elements and the connective tissue undergo the same 

 changes as in the decidua parietalis and here also result in the differentiation 

 of a compact layer and a spongy layer. Both layers are much thinner than 

 in the decidua parietalis. 



As already noted, connective tissue septa pass from the superficial layer of the 

 decidua basalis down into the fcetal placenta subdividing the latter into cotyle- 

 dons. At the margin of the placenta the decidua basalis passes over into the 



Remnant of syncytium 

 Capillaries * 



Nuclear group 

 Artery — , 



Capillary 



Nuclear group 



Fig. 112. — Transverse sections of chorionic villi at the end of pregnancy. Schaper. 



thicker decidua parietalis and here the chorion is firmly attached to the decidua 

 basalis. 



There still remains to be considered what may be called the border zone 

 between the decidua basalis and the chorion frondosum. The whole purpose 

 of the placenta is the interchange of materials between the maternal and fcetal 

 circulation. It is in the border zone that this interchange takes place. The 

 entire structure of this zone is for this function, while all the rest of the placenta 

 serves to transport the blood to and from this area. We have considered on the 

 maternal side the structure of the superficial (compact) layer of the decidua 

 basalis (p. 123), on the fcetal side the structures of the villous layer of the chorion 

 frondosum (p. 124). Unfortunately, this border zone has an extremely com- 

 plicated structure which is difficult of interpretation in the usual microscopic 

 section. This has led to much confusion in description and many differences 

 of opinion as to actual structure. We can here consider only the more generally 



