196 Abthur Willey, 



qualities or altering its general significance, so a chorion may vary 

 in its cornposition without ceasing to be an effective foetal envelope 

 in immediate contiguity with the uterine wall. 



This digression seemed necessary in view of a certain amount 

 of conflict of terms which is undeniably present in the literature of 

 the subject. What is this huge bladder-like foetal vesicle which 

 one extracts from the utricle (uterine Chamber) that contained it? 

 There is no available name for it unless we employ the word 

 blastocyst, neither in a strictly physiological nor in a morpho- 

 logical sense, but in a conventional sense. In the descriptive part 

 of this paper. by „blastocyst" I mean the blastocyst of the beaver at 

 the particular stage when it feil into my hands; by ..chorion", the 

 outer wall of this blastocyst which occurs normally in close juxta- 

 position with the uterine wall; and by „umbilical vesicle" is meant, 

 not „yolk-sac", but a structure resulting from the transformation 

 of the primitive blastocyst. 



The enormous extension of the blastocyst in the later stages 

 seems to indicate that a deeper meaning resides in this formation 

 than the loss of an accumulation of yolk in the egg; in other words 

 it should denote a progressive reaction rather than a passive re- 

 capitulation. Hence arises the necessity for an objective termino- 

 logy. in order to avoid the incessant introduction of the yolk-problem. 



4. The fresh blastocyst. 



Judging from the condition of the membranes and of the ute- 

 rine wall, the size of the foetus, the bulk of the placenta, and the 

 ease with which the placenta came away from the decidua se ro- 

 tin a. together with the published Information (Seton) that the 

 mating season of the beaver falls in February, that the period of 

 gestation is about three months, and that the 3 r oung are born in 

 May. the foetus obtained by me must have been approaching the 

 term of gestation. Yibrissae are present at the sides of the snout 

 and sensory hairs on lips and chin. The head and nape are covered 

 with a very thin layer of fine brownish hair through which the skin 

 is clearly seen. The rest of the body is naked, though the be- 

 ginnings of the hair-tracts can be seen with a lens, and minute 

 white specks are scattered all over the body at intervals. 



Lewis Moegan says that the skin of the foetal beaver, of which 

 he had two specimens in Ins collection, „is covered with a thick 



