173 



There are 110 glands in the placental region, and the uteriue 

 epitbelium is believed to have disappeared. 



Insectivora. 



It is to Hubrecht that we are indebted for a very large pro- 

 portion of the works that have appeared on the placentation of 

 this group. He has been personally responsible for meinoirs on 

 Erinaceus (20), Sorex (21) and Tupaia (22); while Strahl (44) and 

 Vernhout (52) have given descriptions of the placenta oftheMole. 



a.) Erinaceus. 



The placenta is in the form of a concave disc on the anti- 

 mesometric side of the uterus. The blastocyst is imbedded in a 

 'decidua reflexa' which is homologous with that structure in man. 



The embryonic pole faces away from the mesonaetrium. At the 

 other pole there is a large yolk-sac, with a well-developed area 

 vasculosa which persists till a very late stage; the yolk-sac itself 

 is however gradually forced away from the (omphaloidean) tro- 

 phoblast by the growth of the embryo and the allantois. The 

 last-mentioned is a large hollow sac ; many of its blood vessels 

 pass straight across its cavity. 



As soon as the blastocyst is enclosed in the groove of the 

 'decidua reflexa' (which is formed by two thick folds of the anti- 

 mesometric uterine wall) the lips of the latter meet and comple- 

 tely shut in the embryo; at the same moment a clot of blood 

 is formed at this point by extravasation from the neighbouring 

 blood-vessels. 



The trophoblast is from the first exceedingly thick, and, as 

 soon as the epithelium which lined the cavity of the 'reflexa' 

 has disappeared, extravasated maternal blood makes its way into 

 lacunae which are excavated in it; the anti-mesometric (allan- 

 toidean) portion of the trophoblast is subsequently vascularized 

 by the capillaries of the allantois, and by the continued centri- 

 petal growth of capillaries and trophoblast together the placenta 

 is formed , and au intimate relation between the two circulations 

 established. 



