440 THE EABBIT. 



or uterine capillaries. At the same time the inner or deeper 

 surface of the ectoplacenta is becoming folded, and the meso- 

 blast of the somatopleure grows in between these folds (of. Pig. 

 147, e'). The uterine glands of the placental lobes have by this 

 time almost disappeared (c/. Fig. 169, Gw) ; and a little later they 

 are completely absorbed. In the submucous layer the perivas- 

 cular thickening of the walls of the capillaries has proceeded still 

 further, while the capillaries themselves are even larger than 

 before ; and ultimately nearly the whole of the connective tissue 

 of the submucous layer becomes converted into perivascular, or, 

 as they are now termed, decidual cells. 



In the periplacental lobes somewhat similar changes occur. 

 The superficial epithelium of the uterus, and the epithelium 

 lining the mouths of the glands degenerate and disappear ; and 

 perivascular thickenings of the capillary walls occur, although 

 to a less marked extent than in the placental lobes. 



In the obplacental region also, the uterine epithelium de- 

 generates and becomes absorbed ; but the epithelium of the 

 glands themselves remains, and at a later stage, by spreading 

 outwards from the mouths of the glands, reconstitutes an epi- 

 thelial lining to this part of the uterus. 



Towards the end of the tenth day, and during the eleventh 

 day, the allantois is growing rapidly. As shown in Figs. 146 

 and 147, the outer or mesoblastic wall of the allantois very early 

 coalesces with the mesoblast of the outer layer of the amnion, 

 opposite the placental area ; and in this way the blood-vessels 

 of the allantois are brought immediately beneath the ectopla- 

 cental epithelium, and consequently into close jDroximity with 

 the dilated capillaries of the iiterus : and the placenta is thus 

 established. 



The further development of the placenta consists mainly 

 in a gradually increasing complication and elaboration, by 

 which folds of the mesoblast, containing the allantoic vessels, are 

 carried deeply into the ectoplacenta from its inner surface ; 

 while from the outer surface the maternal vessels extend in 

 farther, and in larger numbers, than before. This interdigita^ 

 tion of the foetal and maternal blood-vessels is accompanied by 

 progressive thinning of the layer of ectoplacental epithelium 



