FCETAL NUTRITION: THE PLACENTA 409 



chorion, and fit into vascular crypts in the uterine mucosa 

 from which they are easily retracted at birth. The epithelium 





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of the crypts persists as in the cow, and the " osmotic inter- 

 change takes place through two cell-layers of different origin, 

 and of different physiological significance (phylogenetically). 



