398 THE PHYSIOLOGY OF REPRODUCTION 



fit into depressions or crypts on the surface of the cotyledons, 

 increase in length, and branch in different directions. Whether 

 they literally grow into the maternal tissues either mechanically 

 or by a phagocytic action is uncertain. 1 It seems more likely 

 that very little, if any, further penetration occurs, but that the 

 sub-epithelial tissue swells and keeps pace with the villi as they 

 increase in length. The crypts, if their lining cells really belong 

 to the foetal ectoderm, are not secretory, 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- 



tr 



str 



FIG. 90, Section through the uterine and embryonic parts of a cotyledon 

 of the sheep at the twentieth day of pregnancy. Folds in the tropho- 

 blast fitting into sulci of the cotyledonary burr. (Assheton.) 



mes, mesoblast ; tr, trophoblast ; us, degenerated uterine epithelium ; 

 str, uterine stroma. 



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 cotyledonary interdigitation. At the fundus 

 of the crypts the lining cells 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. 91). 



1 At this stage Assheton did not observe any actual engulfment of cells, 

 but considered that nutriment might be transmitted by fine processes of the 

 binucleate cells which united with similar processes of the connective tissue 

 cells of the mucosa. 



