Foetal Membraues of the American Beaver (Castor canadensis). 195 



lield opinion that the umbilical vesicle is the recapitulation of a 

 yolk-sac. 



A glance at the literature is enough to prove that this is a 

 question involving many complications and that the umbilical 

 vesicle is by no means the obvious relic of a yolk-sac ; and to desig- 

 nate it simply as the yolk-sac (Dottersack) as so often happens in 

 text-books, seems to be a begging of the question. What appears 

 obvious in this matter is clearly superficial and does not touch the 

 root of the problem. 



The mammalian blastocyst is a formation sui generis without 

 the wide significance which is conveyed by the term blastula. 

 The physiological character of the blastocyst is retained throughout 

 the period of gestation, commencing with the primary blastodermic 

 vesicle. then passing through an intermediate phase and culminating 

 in the definitive blastocyst represented by the chorion enveloping 

 the foetus. Both the wall and the cavity of the blastocyst undergo 

 a series of transformations and substitutions which do not alter its 

 essential nature. Just as the embryonic disc produces the mature 

 foetus, so does the primary blastocyst produce the definitive blasto- 

 cyst, and it is therefore convenient to apply the same word to the 

 entire foetal formation from the beginning to the end of gestation. 



Tims the main wall of the blastocyst is at first formed by the 

 trophoblast (Hubeecht); then it may be formed by a non-vascular 

 bilaminar (ectoderm and endoderm) omphalopleure; and again to a 

 large extent, varying according to the particular types under in- 

 vestigation, by a vascular omphalopleure (J. P. Hill); or, as in 

 Carnivora, by the somatopleure of the exocoelom. Similarly the 

 cavity is at first the cavity of the blastodermic vesicle or primary 

 trophocoel; then it may become the cavity of the umbilical vesicle 

 or primary gastrocoel ; and lastly the main cavity of the blastocyst 

 may be represented by the exocoelom. 



The classical meaning of the term chorion is the membrane 

 which encloses the foetus in the uterus, and it seems desirable that 

 this general meaning should be retained for it. Different parts of 

 it at different periods and in different types, may reccive, as they 

 have received, different names; bat the old expressions „true chorion" 

 and „false chorion", like „true amnion" and „false amnion", no 

 longer satisfy the requirements of out knowledge. Just as a 

 rain-coat may be made of very different materials, ranging 

 from straw to oil-skin and rubber, without losing its Avaterproof 



18* 



