AND THOSE OF CERTAIN OTHER MAMMALIA. 81 



The prolongation of decidua serotina, or, in other words, the de- 

 cidua reflexa, does not entirely encapsulate the ovum, as we shall 

 see that it does in certain rodents, but falls short of doing- so by 

 an interval homologous with the non- vascular umbilicus-like "spot 

 observable in the decidua reflexa of the human subject 1 . The 

 entire structure inter-utero-placentally and extra-ovularly placed 

 is bell-shaped, as compared with its wafer-shaped homologue in the 

 tenrec, or its closed sac-like homologue in the early embryo of the 

 rat. The decidua reflexa is sometimes spoken of as being an out- 

 growth of the decidua serotina 2 , sometimes as being continuous 

 rather with the decidua vera in the human subject 3 . What is of 

 importance to bear in mind is, that the non-placental mucous 

 membrane becomes continuous at the periphery of the placental 

 area with both non-deciduous and deciduous serotina ; and that from 

 the point of its junction with this latter structure a more or less 

 extensive envelope grows outwards over the chorion to which the 

 name ( decidua reflexa' is given. The placenta at this period in 

 the hedgehog is a much smaller structure than the deciduous 

 serotina it underlies ; it is itself underlain by a purse-shaped 

 allantois like that of the rabbit, which moors the amniotic sac and 

 the foetus it contains to one pole, whilst the yelk-sac moors it to 

 the other pole of the chorion. As in the early foetus of the dog, 

 so in that of the hedgehog at this period, it is possible by macera- 

 tion to separate entirely, or nearly so, the foetal villi growing on 

 the allantoic area of the chorion from the maternal elements, 

 which shortly after become inextricably interfused with them to 

 form the 'placenta.' 



In a shrew (Sorex sp. ?) the foetus being at an early stage of 

 development, and only three-eighths of an inch long, the deciduous 

 serotina was, as in the hedgehog, about three times the size of the 

 placenta proper. I did not satisfy myself that it was prolonged 

 into a decidua reflexa ; that it was so is rendered improbable by 

 the fact that the other membranes were drawn out at either end 

 into slender tubular processes of an eighth of an inch in length, 

 beyond the sub-globular space occupied in the uterine cornu by 

 the foetus. Its structure was coarsely columnar; that of the 

 placenta presented a villous appearance. The placentae were 

 attached to the free border of the uterine cornu. 



1 Kblliker, 1. c. pp. 178 and 181. 2 Ibid. p. 142. 3 Kobin, I. c. p. 131. 



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