CHAP, ii.] PREGNANCY AND BIRTH. 379 



The placenta then, taken as a whole, presents in the first place 

 a mechanical arrangement by which the foetal blood carried to the 

 villus by the umbilical artery is brought in an ample manner into 

 close proximity to the maternal blood carried to the intervillous 

 space in a very direct way by the curling uterine arteries. But 

 this is not all. The partition between the foetal and the maternal 

 blood is not an inert membrane serving a mechanical purpose only ; 

 the epithelium of which it is in part composed exerts, we must 

 believe, an important influence on the interchange between the 

 foetus and the mother. Moreover the decidual layer consists of 

 other also apparently active cellular structures, which we may 

 conclude exert an influence on the maternal blood as it passes 

 through their midst on its way to and from the intervillous 

 spaces, as also on the blood during its stay in the intervillous 

 spaces which adjoin the decidual layer. In the early stages of 

 embryonic life, before the metabolism of the embryonic tissues 

 has become specialized, this part of the placenta is prominent, 

 it forms a large portion of the whole attachment of the embryo to 

 the mother and is obviously the seat of important changes, one 

 object of which appears to be the nourishment in a special manner 

 of the embryonic tissues. In the later stages, as the foetus 

 becomes more and more capable of transforming for its own uses 

 food of a more common kind, this part of the attachment loses its 

 predominance, and is reduced to the decidual layer having the 

 structure we have described. This however remains to the end 

 of intra-uterine life and assists the epithelium of the villi in the 

 metabolic labours whereby the embryonic blood is adequately 

 nourished at the expense of that of the mother. 



954. Jt appears then that in the transformation of the 

 decidua into a placenta all obvious traces of the glands, unless it 

 be what we have called the basal remnants, have disappeared ; the 

 uterine mucous membrane has been replaced by the decidual layer, 

 and by the system of blood sinuses into which the foetal villi 

 project. We may in the fate of the uterine glands in pregnancy 

 trace a certain analogy with what takes place in menstruation. In 

 menstruation there is an enlargement of the uterine glands, which 

 appears to have for its object an increased secretory activity ; this 

 is followed by a shedding of portions of the membrane, sometimes 

 of the whole of it with the exception oi the basal portion, from which 

 new growth takes place : and we may look upon this shedding as a 

 violent act of secretion. In pregnancy a similar but more marked 

 enlargement amounting to considerable hypertrophy at first takes 

 place, and this too we may perhaps regard as having for its object 

 increased secretory activity, destined in this case for the nutrition 

 of the embryo. In this activity the whole decidua at first shares, 

 the influence of the decidua reflexa being direct, that of the vera 

 more indirect ; but very early the nutrition of the embryo is con- 



