158 R. J. Harrison and J. L. D'Silva 



is frequently to be observed a narrow layer of pyknotic, 

 heavily staining nuclei separating the chorionic cells from the 

 maternal stroma, not only in the region of the membranous 

 chorion (PL II, Fig. 1) but also in the placentome (PL II, Fig. 5). 

 This layer may represent the attenuated and dying maternal 

 epithelium, but it is also possible that it is the remnants of 

 foetal cells which have invaded and destroyed the maternal 

 epithelium and have themselves subsequently died (Harrison 

 and Hamilton, 1952). 



Trophoblastic giant cells, possessing the histochemical 

 characteristics so clearly described by Wimsatt (1951) in the 

 sheep, are observable at the edges of the villi (PL II, Fig. 3). 

 Similar cells can also be observed in situations which strongly 

 suggest that the giant cells pass across and become inter- 

 calated within or even replace the maternal epithelium. It is, 

 however, difficult to decide whether these cells can exist for 

 very long in their close relationship to the maternal stroma. 



Acknowledgements. 



The authors express their thanks to Dr. F. L. D. Steel and Mr. C. J. 

 Turner for their help with the experimental work, to Mr. R. Q. Cox and 

 Mr. R. F. Birchenough for technical assistance, and to the Agricultural 

 Research Council and the Yarrow Fund of the London Hospital Medical 

 College for grants to defray expenses incurred in purchasing animals and 

 radioactive isotopes. 



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