Lnternal Factors of Sex Determination 419 
tion that rests on the assumption that the chromosomes that 
arise from a female individual have a greater tendency to pro- 
duce a female; and those that originate from a male individual 
have a greater tendency to produce a male. Since the child 
gets as many chromosomes from the father as from the mother, 
the parental chromosomes as such cannot determine the sex. 
But it is to be recalled that amongst the parental chromosomes 
some have come from the grandfather and some from the 
grandmother. The relative number of chromosomes from the 
maternal and paternal lines will be variable in number on the 
current assumption that at the reduction division it is merely a 
question of chance which member of a pair of homologous chro- 
mosomes goes to one pole of the spindle and which to the other. 
If the chromosomes of the grandfather predominate in the 
offspring, it will be a male; if the grandmother chromosomes 
predominate, a female develops. 
To take an example. If the somatic number of chromosomes 
for the human species be assumed to be 24, the child gets 12 
from the father and 12 from the mother. If amongst the former 
there are 8 grandmother chromosomes and amongst the latter 
7 grandmother chromosomes, the child will be a girl, for there 
are at least 15 of the 24 derived from the grandmother’s 
side. ° 
On Ziegler’s theory of sex it is evident that whenever the re- 
duced number of chromosomes is even, there may occur an exact 
balance of grandmother and grandfather chromosomes, hence 
the child can have no sex at all. In the list of cases given by 
Ziegler himself, we find that the reduced number of the chro- 
mosomes is an even one in 29 species and odd only in ro. 
It seems improbable that the equal balance of the maternal 
and paternal chromosomes could be counterbalanced by the 
presence of chromosomes derived from the grandparents, espe- 
cially since these have also been contained in one or the other 
parent whose sex, on the theory, should have influenced them to 
acquire the character of that parent. These and other diffi- 
culties make Ziegler’s hypothesis very improbable. 
