Nachdruck verboten. 

 Ubersctzunysrecht vorbehalten . 



Foetal Membranes of the American Beaver (Castor 



canadensis), 



By 



Arthur Willey 



(McGill University, Montreal). 

 With 10 figures in the text. 



1. Introduction and acknmvledgnients, 



Classical researches such as those of Selenka, Düval, and 

 Fleischmann, have shown that the study of the placentation even 

 of closely related Rodents is likety to reward the investigator with 

 a number of details of considerable novelty and interest. 



So far as I am aware there has been no published description 

 of the intra-uterine stages of the beaver, and although my material 

 consists of one stage only, and that a late one, yet it exhibits 

 certain particularities which may not be without interest in spite 

 of the lack of earlier material leading up to it. The risks of error 

 in attempting to describe and above all to Interpret a haphazard 

 phase of development are very great. and only the rarity of the 

 material and the difflculty of collecting it, may be held to justify 

 the attempt. Undoubtedly it would have been better to have de- 

 voted years to the accumulation of the material so as to be able 

 to arrange the successive stages in an unbroken series, but this 

 procedure is not always practicable. Moreover, thanks to the clas- 

 sical monographs which have been published since the time of 



