Foetal Membranes of the American Beaver (Castor cauadensis). 211 



Three coincidences seemed to me to be worth noticing, namely, 

 the exclusive capture of feraales, the unilateral gestation, and the 

 male sex of the two foetus taken from the body of the second 

 pregnant female. These facts appeared to indicate a possible segre- 

 gation of sexes in the beaver, extending to the habits of the adult 

 and the gestation of the young. 



On my return to Montreal I took steps to verify the behaviour 

 of rabbits in regard to the distribution of sexes in the horns of the 

 uterus. I ascertained that rabbits are ambidextrous, if that ex- 

 pression suits the case; in other words, that male and female foetus 

 occur indifferently in the right and left uterus. I made this out by 

 dissection of the foetus extracted from the uteri, controlled by 

 sections of the foetal gonads, not by extirpation of one of the ovaries 

 of the parent. 



While this experiment with rabbits was going forward I became 

 aware, from the columns of „Nature*' (May 18th, Vol. 86, p. 392) of 

 the existence of Dr. Rumley Dawson's book on The Causation 

 of Sex (1909), and of the paper on the effects of ovariotomy in the 

 rat by L. Doncaster and F. H. A. Marshall (1910). Dr. Dawson's 

 hypothesis supports the view that in the human species the right 

 ovary gives rise exclusively to male-producing ova and the left 

 ovary to female-producing ova. The theory is based largely upon 

 evidence of alternate male and female ovulations. 



Doncaster's and Marshall's experiments on the rat „indicate 

 without any doubt that in the rat it is not true that ova deter- 

 mining one sex are produced from one ovary, and those determining 

 the opposite sex from the other, for each rat, with one ovary com- 

 pletely removed, produced young of both sexes." iThey add that 

 „this does not of course prove that the 'right and left ovary hypo- 

 thesis' is not true for man, bat its definite disproof for another 

 mammal detracts from its probability." 



In view of these Statements I proceeded to determine the 

 sex of the four foetus obtained from the first pregnant female. 

 They all proved to be males, the testes occupying an identical 

 Position in each, far back in the inguinal region of the abdominal 

 cavity. 



The unilateral Ovulation of birds has nothing in common with 

 the unilateral gestation of mammals, except in regard to the physico- 

 mechanical or topographical conditions which accompany it, if indeed 

 they do not in some measure determine it. The situs matricis 



14* 



