181 



gestation are, briefly, as follows. The epitheliuui at the surface 

 becomes thin, and disappears. The epithelium of the glands and 

 crypts first hypertrophies and then degeuerates ; the process begins 

 at the surface and gradually advances inwards, stopping short, 

 however, of the deep, blind ends of the crypts. The latter, together 

 with those portions of the ducts of the glands which are at the 

 same level, becorue enortnously dilated, and the thin connective- 

 tissue septa between them, called by Duval the 'lamelies mésentéri- 

 formes', give passage only to the maternal bloodvessels and form 

 the sole connection between the placenta and the wall of the uterus. 

 Immediately at the surface is a layer of closely packed capillaries, 

 with a well-rnarked endothelium. This layer is invaded, and the 

 capillaries completely enveloped by processes of the trophoblast ; 

 a composite tissue is thus produced, the 'angio-plasmode', which 

 when vascularized by the allantois gives rise to the placenta. 

 The first villi which penetrate the 'angio-plasmode' are not 

 vascular ; they are developed solely from the somatopleure. The 

 portions of the 'angio-plasmode' between them are spoken of as 

 'lobules'. These primary villi are however soon reinforced by the 

 bloodvessels of the allantois, which advance into the lobules, and 

 cut them up into very numerous lamellae, which run in all 

 directions. In the latest stages of the placenta the lamellae, consist 

 of 1) maternal bloodvessels with persistent endothelium, 2) outer 

 and inner unclear layers, derived from the trophoblast, and pene- 

 trated, now, on the outside by 3) the minutest capillaries of the 

 allantois. 



The whole of the 'angio-plasmode' is however not subdivided 

 in this manner; certain tracts are left. for the passage of large 

 maternal arterial bloodvessels, which, as we have already had 

 occasion to notice in other forms, pass straight through to the 

 foetal side of the placenta before breaking up into capillaries ; 

 this arrangement had been previously described by Tafani. It 

 is important to observe that the increase in thickness of the 

 placenta is not due to a progressive invasion of maternal by foetal 

 tissues, but to au independent growth of the 'angio-plasmode' 



