FOETAL NUTRITION: THE PLACENTA 



431 



and there is no free space, such as is described in the mare, between 

 them and the villi. The sub-epithelial tissue is represented in the 

 non-pregnant uterus by a thin layer of dense connective tissue, with 

 localised thickenings in the burrs. With the onset of pregnancy 

 occur an infiltration of lymph between the more superficial cells 

 of the sub-epithelial layer, and an increase in the number and size 

 of the blood- capillaries and lymphatics. Thus the layer becomes 

 spongy and swells up around the foetal villi, producing the cotyle- 

 donary interdigitation. At the fimdus of the crypts the lining cells 



m.v 



Tr 



FIG. 116. Section through the base of a foetal villus and the apices of two inter- 

 crypt columns. Sheep. The surfaces of the columns are traversed by 

 large blood-vessels, which later rupture and form the blood-extravasations. 

 (Assheton.) 



Tr, Trophoblast dipping into crypt ; Ic, inter-crypt column ; m.v, maternal 



blood-vessel. 



become syncytial. At the apices of the inter-crypt columns lacunae 

 of maternal blood are formed by repeated small haemorrhages from 

 the superficial capillaries (Fig. 116). 



In the inter-cotyledonary area, the epithelium, whether or not it 

 degenerates over large areas in the early stages as Assheton supposes, 

 is later healthy and vigorous. There is no formation of a spongy 

 layer in the sub-epithelial tissue as in the burrs. But a great change 

 occurs in the glands, which are wholly inter-cotyledonary in position. 

 They increase in length and complexity, and secrete actively. Towards 

 the end of pregnancy, however, the greater part of the uterine glands 

 is destroyed, but the surface epithelium still secretes. At the upper 

 end of each horn the wall of the blastocyst forms a crumpled 



