175 



Mesometrically again to the area vasculosa is another ring- 

 shaped thickening, formed however from the trophoblast alone, 

 which is not here attached to the uterus. This is the 'tropho- 

 blastic anuulus'; it cousists of coluranar cells which iugest extra- 

 vasated ruaternal blood corpuscles. Apparently as a result of this 

 process a green pigment (presumably biliverdin) is found in the 

 cavity of the yolk-sac. As pregnancy advances the trophoblastic 

 anuulus is found closely attached to the edge of the growing 

 placenta. 



Tn the placental region there is, at the cominencement of 

 gestation, an intense proliferation not only of subepithelial tissue 

 but also of the maternal epithelium. The result of the latter 

 process is the formation of a number of shallow epithelial crypts, 

 which are subsequently penetrated by thick knob-like processes 

 of the allantoidean trophoblast. Directly this has occurred, the 

 epithelium, both on the surface and in the crypts begins to dege- 

 nerate, and a similar degeneration affects the glands in the later 

 stages. 



The placenta, is formed by the excavation of lacunae in the 

 trophoblast, in which maternal blood circulates, and by the vas- 

 cularisation of the trophoblast by capillaries of the allantois; and 

 it is extended by the centripetal growth of both these tissues. 

 In the trophoblast a cytoblast may be distinguished from a plas- 

 modiblast. The latter grows at the expense of the former. 



c.) Tupaia. 



In Tupaia there is a doublé placenta; the halves are discoid 

 and situated on opposite sides of the uterus. It is formed from 

 the trophoblast in essentially the same way as in Erinaceus and 

 Sorex; at first it is vascularised by the area vasculosa but this 

 is subsequently replaced by the allantois. The uterine epithelium 

 degenerates, and the trophoblast is divisible into cellular and 

 plasmodial layers. It is of interest that the situations of the two 

 halves of the placenta are marked out, long before the fixation 

 of the blastocyst, by two 'Haftflecke'. These 'Haftflecke' are areas 

 of intense subepithelial proliferation ; and as a result of the process 



