194 Arthur Willey, 



the picro-nitric Solution, and in a sublimate-acetic-formalin mixture 

 (concentrated Sublimate 3 parts, glacial acetic 1 part, 100 cm 3 ; 10 

 per cent formalin 100 cm 3 ). The last-named mixture was used chiefly 

 for the fixation of other portions of the beaver's anatomy. 



For histological preparations to control the main observations, 

 pieces were stained in toto with Grenacher's alum carmine, and 

 in section with Delaeield's alum haematoxylin and Heidenhain's 

 iron haematoxylin. This work has been carried out during the long 

 vacation of 1911 in the Department of Zoology of McGill University, 

 Montreal, Canada. 



Incidentally I may mention that Trematode parasites were found 

 sparsely in the coecum and large intestine of some of the beavers. 

 Thompson Seton states that beavers are remarkably free from in- 

 ternal parasites. Nematodes were also present. 



Under hard conditions of travel there can be no doubt that 

 formalin and Sublimate are the most economical and efficient fixa- 

 tives, the latter especially being the reagent most suitable for the 

 satisfactory preservation of Trematodes. 



3. Terms employed in the description. 



Before recording my observations on the freshly excised speci- 

 mens, it is desirable to justify the use of the term blastocyst 

 as applied to the entire embryo in its chorion at a late period of 

 gestation. The term chorion itself will also require some ex- 

 planation in order to avoid possible confusion, due to the fact that 

 the chorion of the mammalian embryo is not always the same thing, 

 just as the placenta. as pointed out by Düval, is not a uniform 

 structure throughout the mammalian series. Blastocyst, chorion, and 

 placenta in mammals, are structures developed in connection with 

 the peculiar relations established between the embryo and the wall 

 of the Uterus. According to views suggested both b} r Hübrecht 

 and by Assheton, the form of the mammalian blastocyst is deter- 

 mined in the first instance by the reaction to the Stimulus of an 

 intra-uterine environment, not by the force of heredity which would 

 compel the embryo to recapitulate structures formed originally in 

 connection with an accumulation of yolk. I suppose it is not possible 

 to ignore the existence of such a Stimulus, but I do not propose to 

 discuss at any length the controversial question as to whether the 

 response to that Stimulus is or is not compatible with the widely 



