179 



In Hylobates (and also apparently in Simia) there is a 'decidua 

 reflexa' as in Man. The placenta is single, and discoidal, but 

 Selenka has sliown that at an early date there is an area of 'villi' 

 at the non-placental end of the blastocyst, and a few sparsely 

 scattered 'villi' in between. The 'villi' are covered by a syncytium 

 to which Selenka attributes a phagocytic function. The 'decidua 

 reflexa' degenerates later. 



In the other two forms mentioned there is no 'reflexa'. In 

 Semnopithecus the placenta is single, in Cercocebus it is doublé, 

 there being two areas of 'villi' at opposite poles of the blastocyst. 



Selenka does not commit himself definitely to the belief that 

 the syncytium covering these villi is erabryonic in origin ; he 

 points out, in fact, that other syncytial masses (Nestensyncytia) 

 are present which are clearly derived from the uterine epithelium. 

 From what we know, however, of the development of the syncy- 

 tium iu Tarsius and Man, it would seem to be very probable 

 that in Monkeys also it is trophoblastic. 



c.) Man. 



The general relations of the placenta and foetal membranes in 

 man are so well-known that it is uunecessary to enter into them 

 at any length here. I only wish to draw attention to those facts 

 which appear to me to be of the greatest importance. 



The best theory of the formation of the 'reflexa' seeras to be 

 that the blastocyst sinks into the subepithelial tissue through a 

 gap due to the degeneration of the epithelium ; if so there can 

 be no question of the persistence of uterine epithelium in the 

 region of the placenta. In this region, the 'serotina', there is, at 

 the commencement, an oedematous proliferation of connective tissue, 

 accompanied by extravasatiou of blood, dilatation of the blood- 

 vessels and degeneration of the ducts of the glands. As gestation 

 advances the serotina grows thinner ; this is due to the degeneration 

 of the layer known as the compacta ; at birth the break takes 

 place aloug the outer border of this layer. 



The branching allantoic villi are covered by two layers, firstly, 

 a layer of cells the so called 'Langhansche Zellschicht', and, secoudly, 



