210 Arthur Willey, 



mesenteric cord, on leaving the common umbilical cord, ascended to 

 the chorion on the right side of the foetus instead of on the left. 



6. Unilateral Gestatio n. 



The material upon which this paper is based comprised alto- 

 gether six adults and six foetus. Although the adults, caught in the 

 traps, were of different ages, from yearling upwards, they were all 

 females. The yearling beaver is regarded as a delicacy and the 

 specimen had been cut up into „petits morceaux u before I had an 

 opportunity of dissecting it; but my companion, an intelligent man 

 and a great connoisseur of game. assured me positively as to 

 its sex. 



According to E. T. Seton and to common report, the beaver 

 does not begin to breed until it is three years old and it attains 

 the age of 12 — 15 years (Moegan). Seton says that the "beaver 

 is a strict monogamist. The mating season is February, and the 

 pair make then and there a contract for life. Gestation lasts about 

 three months; toward the end of that time the mother separates 

 herseif from her mate, that is, I suppose, compels him to move out 

 and keep away. . . . The young are born in the month of May ; their 

 eyes are open from the first." 



The first pregnant female obtained by me, taken from the trap 

 on April 21 st , contained, as mentioned above, four advanced foetus 

 in the right uterus only; these were examined as to their membranes 

 in the fresh condition and were then preserved without further 

 dissection. The left uterus was empty, thick-walled, of uniform dia- 

 meter; and subsequent sections showed it to be in a normal, healthy 

 condition, with uterine epithelium intact. The left ovary presented 

 the same general appearance as the right ovary dotted here and 

 there with small round clear patches, the follicles. 



The second pregnant female was taken on the following day. 

 It contained two equally advanced foetus in the right uterus only; 

 the left uterus was empty as in the previous example. I dissected 

 these on the spot and, having ascertained that they were both males, 

 cut out the testes and fixed them, one pair in concentrated Subli- 

 mate Solution, the other pair in Flemming's Solution, leaving the 

 vasa deferentia in situ in the bodies of the foetus, which were then 

 preserved. Each foetus possesses four very distinct mammary 

 pits in the pectoral region. 



