RETAL NUTRITION: THE PLACENTA 465 



was absent (Selenka). Spaces, which are in direct com- 

 munication with maternal capillaries, are present in the 

 syncytium. The most notable characteristic in the decidua is 

 the presence of a glandular secretion in the embryo trophe. In 

 the non-placental area the glands are dilated and open into the 

 uterine cavity, many of them close to the peripheral villi. 

 Hence their secretion may reach the trophoblastic lacunae. In 

 the placental region they are also dilated, but their superficial 

 parts are closed and appear to degenerate early. In the decidua 

 he nests of epithelioid cells, the origin of which is uncertain. 



The new-world monkeys, like the old-world, have no de- 

 cidua capsularis, and the placenta is formed as a single disc. 

 In the anthropoid apes, on the other hand, the ovum is lodged 

 in an implantation cavity, and so is covered by a reflexa. The 

 whole circumference of the trophoblast thickens and develops 

 villi, but later they disappear except over a discoid area, the 

 decidua serotina. In the earlier stages two main groups of 

 villi are present, as in the old-world monkeys, while the rest of 

 the chorion is covered with smaller villi. 



In Selenka 's youngest specimen, the ovum was completely 

 enclosed by decidual tissue, and there was no evidence to show 

 whether the mode of embedding was excentric or interstitial. 

 The surface of the ovum was separated from the decidua by a 

 series of intercommunicating spaces, the intervillous spaces, 

 which contained lymph. In other words, Selenka looks on the 

 intervillous space in apes as a space lying between maternal 

 and foetal tissues, in which villi are suspended. 



In Man also the villi are at first diffuse, and later restricted 

 to a discoid area, the placenta being again developed in the 

 decidua serotina. 



The ovum probably reaches the uterus still enclosed in the 

 zona pellucida, and lies free until the end of the first week, but 

 this stage has never been observed. The uterine mucosa, as in 

 other orders, is matured about the time of puberty (Bjorken- 

 heim 1 ), and then consists of embryonic connective tissue cells, 

 separated from the surface epithelium by a layer of flattened 

 cells. The intercellular spaces are filled with lymph, and they 



1 Bjorkenheim, " Zur Kenntnis der Schleimhaut im Uterovaginalkanal 

 des Weibes in den verschiedenen Altersperioden," Anat. ffefte, H. cv., 1907. 



2G 



