MAMMALIA. 



243 



The blastodermic vesicle of the (Juinea-pig becomes completely en- 

 veloped in a capsule of the uterine wall (decidua reflexa) (fig. 150). The 

 epithelium of the blastodermic vesicle in contact with the uterine wall is not 

 epiblastic, but corresponds with the hypoblast of the yolk-sack of other 

 forms, and the mesoblast of the greater part of the inner side of this 

 becomes richly vascular (yfc) ; the vascular area being bounded by a sinus 

 terminalis. 



The blastodermic vesicle is so situated within its uterine capsule that the 

 embryo is attached to the part 

 of it adjoining the free side of 

 the uterus. From the opposite 

 side of the uterus, viz. that to 

 which the mesometrium is at- 

 tached, there grow into the wall 

 of the blastodermic vesicle a u_ 

 numerous vascular processes 

 of the uterine wall, which es- 

 tablish at this point an organic 

 connection between the two 

 (pi). The blood-vessels of the 

 blastodermic vesicle (yolk- 

 sack) stop short immediately 

 around the area of attachment 

 to the uterus ; but at a late 

 period the allantois grows to- 

 wards, and fuses with this area. 

 The blood-vessels of the allan- 

 tois and of the uterus become 

 intertwined, and a disc-like 



FIG. 150. DIAGRAMMATIC LONGITUDINAL 

 SECTION OF AN OVUM OF A GUINEA-PIG AND 

 THE ADJACENT UTERINE WALLS AT AN AD- 

 VANCED STAGE OF PREGNANCY. (After Bischoff.) 



yk. yolk-sack (umbilical vesicle) formed of 

 an external hypoblastic layer (shaded) and an 

 internal mesoblastic vascular layer (black). At 

 the end of this layer is placed the sinus termi- 

 nalis ; all. allantois ; //. placenta. 



The external shaded parts are the uterine 

 walls. 



The cavity of the allantois, if 



placenta more or less similar 



to that in the Rabbit becomes formed (pt). 



developed, vanishes completely. 



In all the Rodentia the placenta appears to be situated on the mesome- 

 tric side of the uterus. 



Insectivora. In the Mole (Talpa) and the Shrew (Sorex), the fcetal 

 membranes are in the main similar to those in the rabbit, and a deciduate 

 discoidal placenta is always present. It may be situated anywhere in the 

 circumference of the uterine tube. The allantoic cavity persists (Owen), but 

 the allantois only covers the placental area of the chorion. The yolk-sack is 

 persistent, and fuses with the non-allantoic part of the subzonal membrane ; 

 which is rendered vascular by its blood-vessels. There would seem to be 

 (Owen) a small decidua reflexa. A similar arrangement is found in the 

 Hedgehog (Erinaceus Europasus) (Rolleston), in which the placenta occupies 

 the typical dorsal position. It is not clear from Rolleston's description 

 whether the yolk-sack persists till the close of fcetal life, but it seems 

 probable that it does so. There is a considerable reflexa which does not, 



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