790 The Condylarthra. [August, 
female pronucleus it is equal in size to the latter. In the case 
when the impregnation is deferred for four hours, the male pro- 
nucleus never becomes so large as the female pronucleus. With 
reference to the effect of the time at which impregnation takes 
place, Asterias would seem to serve as a type” (Balfour, Comp. 
Embryol., Vol. 1, p. 68). 
Thus when impregnation takes place at a very early period, 
since the ovum in its own independent course of development has 
not yet reached the segmenting stage, the immediate effect of the 
union of the female and male elements is a modification of the 
male element by which it is to some extent assimilated in charac- 
ter to the female element. In consequence there is established in 
the fertilized ovum at the outset a relative preponderance of the 
factor of cell-growth, in its developmental tendencies; and this, 
by the theory, determines to the production of the female sex. 
But where impregnation takes place at a late period, when the 
ovum in its development has reached the segmenting stage, its 
modifying action on the male element before the union is com- 
pleted is less ; and in consequence there is established in the fer- 
tilized ovum at the outset a preponderance: of the factor of cell- 
division, which the male element represents ; and this, by the 
theory, determines to the production of the male sex. ' 
It appears, therefore, that the theory of sex and sexual genesis 
that is here proposed, affords a reasonable explanation of the ob- 
served effects of the time of impregnation in determining sai 
And so far as all the known causes on which the determination of 
sex depends are incapable of being equally well explained on any 
other theory, they may be taken as giving support to this theory. 
PIRES EDIE O ET 
THE CONDYLARTHRA. 
BY E. D. COPE. 
teeth of 
i a paper on the homologies and origin of the molar re 
the Mammalia Educabilia, published in March, 1874 
1 Journal of the Academy of Natural Sciences of Philadelphia. vious that 
which I used is as follows: “I trust that I have made it sufficiently o a> ur 
the primitive genera of this division of mammals [Mammalia Ba ee 
guiculata and Ungulata sensu lata] must have been bunodonts with 
plantigrade feet,” 
e nearest approaches to a similar anticipation on the part of 
which I have been able to find, refer to the number of toes only, and 
other naturalis® 
are of restricted 
ie 
‘oq 
a Ot a re 
