438 THE EABBIT. 



ing to Duval, the uterine epithelium of the placental lobes has 

 entirely disappeared, by absorption, though it remains unaltered 

 in the deeper parts of the glands for some time longer : the 

 embryonic epiblast of the placental area is, therefore, in direct 

 contact with the connective tissue of the uterine wall. 



This thickened epithelium of the placental area of the blasto- 

 dermic vesicle is a structure of very great importance, and has 

 been named by Duval the ectoplacenta. It serves in the first 

 instance, as just noticed, to attach the embryo to the uterine 

 wall, and in the later stages it plays a very prominent part in 

 the formation of the placenta. It must be borne in mind 

 throughout the following description that, if Duval's account 

 is correct, the ectoplacenta is entirely of foetal origin, and is not 

 derived, even in part, from the uterine epithelium. This is a 

 point, however, on which difference of opinion obtains ; Strahl, 

 for instance, maintaining that the ectoplacenta is formed by 

 proliferation of the uterine epithelium, and not from the em- 

 bryonic epiblast. 



The embryo normally lies with its long axis coinciding with 

 that of the blastodermic vesicle, and therefore with that of the 

 uterus, so that a transverse section of the uterus cuts the 

 embryo transversely (Fig. 169, XG). The embryo is usually in 

 the middle of the upper surface of the blastodermic vesicle, and 

 lies opposite the deep cleft, pe, between the two placental lobes. 

 The position of the embryo is, however, variable, especially in 

 the earlier stages : it may lie obliquely across the vesicle ; or 

 may, more rarely, lie opposite one or other of the placental lobes, 

 instead of opposite the cleft between them. 



In the submucous layer of the placental lobes important 

 changes occur during the ninth day. The capillaries dilate very 

 considerably (Fig. 169, uc), becoming much larger than the 

 arteries and veins in connection with them. They retain their 

 simple epithelial walls, but thick adventitious perivascular walls 

 are formed outside these by the surrounding connective-tissue 

 cells. These perivascular cells are at first ordinary connective- 

 tissue cells, which increase in number, draw in their processes, 

 and become arranged in layers, two or three cells thick, around 

 the capillaries. This perivascular thickening of the walls of the 

 capillaries occurs throughout the gi-eater part of the submucous 

 layer, but does not affect the capillaries immediately beneath the 



