PLACENTA IN I'EHAMKIjES 129 



formed. ... In many of the ectoderm cells shown in fig. 9 

 the nuclei are also seen to be in various states of disintegration. 

 Many of them stain only slightly ; the nuclear membrane is 

 becoming indistinct, while the chromatin is found l)roken up 

 and diffused in the form of small granules throughout the 

 delicate nuclear reticulum. Eventually the position of the 

 nucleus is only marked by a few straggling irregularly thickened 

 remnants, which finally become diffused through the proto- 

 plasm and lost to view ' (p. 404). [Italics mine.] 



In this stage (embryos 8-8-75 mm.) the ectoderm is repre- 

 sented centrally, ' only by more or less isolated degenerating 

 cells ' (p. 413).' 



In the 12-5 mm. embryo Hill states that ' over the placental 

 area usually single, much degenerated, and deeply-staining 

 chorionic ectoderm cells are still to be found ' (1899, p. 9). 



From the above it appears that one of the causes of the 

 disappearance of the ectoderm is the loss of the chromatic 

 constituents of the nucleus, these being absorbed into the 

 surrounding cytoplasm. 



As to the other possible reasons for the presumed degeneration 

 and disappearance of this ectodermal layer. Hill states further 

 that, in some cases, ' the inner ends of the cells are greatly 

 vacuolated, a fact which suggests that a process of vacuolation 

 may also play a role in the retrogression of the chorionic 

 ectoderm ' (1897, p. 404). 



Further, referring again to this layer, he is ' inclined to 

 believe that the allantoic capillaries, so closely related to its 

 inner surface, are by no means the least active agents in effecting 

 its removal. Of direct fusiofi of the degenerate ectoderm with 

 the miderhjing syncytium there can he no question. All the facts 

 negative such a view ' (1897, p. 404). [Italics mine.] 



From the above it appears that Hill is of the opinion that 

 the layer of chorionic ectoderm disappears, and that the 

 following processes mutually assist in causing this removal : 



(a) Degeneration in situ with or without vacuolation. 



(6) Removal by allantoic capillaries with or without previous 

 degeneration. 



NO. 265 K 



