Foetal Membranes of the American Beaver (Castor canadensis). 213 



a female porcupine which contained a foetus that would certainly 

 have been born within three or four days. It weighed l x / 4 lbs 

 (567 g) and raeasured in total length 11 % inclies (285 mm), the 

 head and body measuring abont 7 8 / 4 inches (just 195 mm). It was 

 densely covered with long, black hair, and the quills on its back 

 measured a little over half an inch (13 mm) in length. The discoid 

 placenta measured 2V 4 inches (57 mm) in diameter." 



How easy it would have been, and how unnecessary it must 

 have seemed, to have mentioned the hörn of the Uterus and the sex 

 of the foetus. It would have constituted a unit record. Of course 

 the sex of the foetus of Rodents and of many other mammals can 

 only by determined by dissection. 



The varied psychic manifestations which are so much in evidence 

 in the manifold works of the beaver — the dam, the lodge, the 

 burrow. the treecutting, and the artificial canal, — indicate a 

 degree of segregation of the association-centres of the brain beyond 

 what may be perceptible in a histological examination. In this 

 matter of neural or psychic segregation, the beaver occupies a position 

 amongst Rodentia comparable with that attained by Man amongst 

 Primates. Chipped wood is as sure a beaver-sign as chipped flint 

 is a human sign. The particularities of placentation, in Beaver and 

 Man, may perhaps be correlated more or less remotely with their 

 psychic developments. Hubrecht (1908) has suggested that the 

 phenomena of placentation are „intimately related to the higher 

 development which characterises the Mammalia as against the lower 

 Vertebrates". There is thus created an a priori possibility that 

 segregation may aifect the sexual conditions of such highly 

 differentiated types. 



The Rabbit and Rat breed three ör four times a year and 

 commence to reproduce at the age of about six mouths. The Beaver 

 breeds once a year and commences to reproduce at the age of about 

 three years. These facts alone indicate ditferences in the sexual 

 conditions of the Beaver as compared with other Rodents; and the 

 question may be put: — In what do these ditferences consist? Per- 

 haps the answer ma} T be in part: — In unilateral sexual segregation. 

 But before any answer could be given categorically, a much greater 

 quantity of records than is at present available, would have to be 

 examined. 



The normal symmetry of the gonads in mammals is originally 

 part of the primitive symmetry of bilateral animals. Modifications 



