474 THE PHYSIOLOGY OF REPRODUCTION 



in the rabbit, and no endothelial proliferation as in the hedge- 

 hog and bat, though the latter may occasionally occur in 

 tubal pregnancy (Webster 1 ). 



The capillaries dilate to sinuses, and new vessels are also 

 formed in the compacts,. Many of them are opened by the 

 trophoblast and perhaps by the mononuclear cells, and gradually 

 more and more blood is effused into the trophoblastic lacunae. 

 In them it does not clot, the syncytium acting as an endothelium, 

 but at a certain stage the blood begins to circulate and continues 

 to do so throughout pregnancy. The gland-ducts are destroyed 

 in the necrotic zone. In the underlying compact zone they are 

 found dilated in the serotina and base of the reflexa, but even 

 in Bryce and Teacher's ovum the epithelium showed signs of 

 degeneration and desquamation. 



With the formation of the space between the ovum and the 

 decidua, a permanent attachment of the two structures is 

 brought about. The development of the villi has already been 

 traced up to the stage when they consisted of simple stalks of 

 mesoblast with a double ectodermal covering. In the core are 

 developed capillary vessels which are continuous with the vessels 

 of the " Haftstiel," and later with those of the umbilical cord. 

 After the excavation of the necrotic zone, some of the stalks 

 reach the decidual surface and attach the ovum to it. At first 

 the attached ends of these primary villi are plasmodial, but 

 later the cytoblast proliferates and forms thick rounded masses, 

 the " Zellsaulen," over which the syncytium disappears. This 

 forms the permanent attachment between the villi and the de- 

 cidual surface,. The spaces between the stalks form the primary 

 intervillous space, which is thus entirely in the plasmodiblast. 

 The primary villi form buds of their three layers which develop 

 into secondary villi. Of these some may also become attached 

 to the decidua, while others hang free in the intervillous space. 

 By a similar process other villi are also developed, till the whole 

 system becomes branched like a tree (Fig. 127). At first they 

 are equally distributed over the chorion, but the villi in relation 

 to the reflexa do not branch so much, and even at the end of the 



1 Webster, Human Placentation, Chicago, 1901. Wade and Watson 

 (Journ. of Obstet. and Oynec. of Brit. Emp., 1908) also state that in tubal 

 pregnancy some of the decidual cells are formed from endothelium. 



