492 THE PHYSIOLOGY OF REPRODUCTION 



non-plaoental area the glands are dilated and open into the uterine 

 cavity, many of them close to the peripheral viUi. 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 decidua 

 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 refiexa. 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 St 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 (Bjorkenheim i), 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 drain into lymphatic vessels 

 in the" outer half of the mucosa, where also the arterioles and veniiles 

 lie. All the blood-vessels in the inner half are capillaries." In all 

 probability the fertilised ovum, during its sojourn in the Fallopian 

 tube and while it Hes free in the uterine cavity, does not influence 

 the structure of the mucosa, and may implant itself at any period 

 during the oestrous cycle (Bryce and Teacher^). But under the 

 abnormal conditions in a tubal pregnancy, the uterine mucosa under- 

 goes a decidual change although no fertilised ovum is embedded in it. 



1 Bjorkenheim, " Zur Kenntnis der Schleimhaut im Uterovaginalkanal des^ 

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



2 Bryce and Teacher, The Early Imbedding and Development of the Human 

 Ovum, Glasgow, 1908. 



