CHROMOSOMKS, HEREDITY AND SEX. 
503 
develop into males ; these extrude one chromosome undivided 
at the single polar division, so that males have five chromo- 
somes. Sexual females are produced from large eggs which 
have a normal equation division, and therefore contain six. 
In spermatogenesis two kinds of spermatids are produced, 
one with three, the other with two cln'omosomes ; the latter 
degenerate, so that all functional spermatozoa have three. 
The fertilisable eggs have a normal reduction, so that the 
female pronucleus contains three, and the fertilised egg, 
therefore, contains six, and completes the cycle by givingrise 
to a ‘^stem-mother.^’ 
The whole scheme is made clearer by the diagram on p. 504, 
in which the “sex-chromosomes” are represented by X. 
The difference between the chromosomes of female-producer 
and male-producer is not represented, since it has not yet 
been satisfactorily elucidated. The word “ egg ” refers to the 
egg before maturation ; in the parth’euogenetic generations 
the chromosome group of the mature egg is, of course, the 
same as that of the individual which develops from it. 
Two points of great importance should be noticed in con- 
nection with these observations. Firstly, it is possible for a 
])olar division to occur in such a way that one chromosome is 
always thrown out with the polar body (male-producing eggs) ; 
it is, therefore, not entirely a matter of chance to which end 
of the spindle an undivided chromosome shall go. This may 
be a fact of very great moment in interpreting some other 
cases. .Secondly, the absence of one X chromosome is not 
the ultimate cause of male-production, since it is predeter- 
mined in some way that some eggs shall extrude X (possibly 
by the loss of one portion of a compound chromosome in the 
previous generation), and that other eggs shall not. But this 
cannot logically be regarded as a proof that the presence or 
absence of X is not the cause of female ness or maleness ; it 
only means that some factor is present which decides whether 
referred back to the stem-mother ; there would be two kinds of stem- 
mothers, one of which produces female-producing offspring, the other 
male-producing. 
VOL. 59, PAKT 4. NE\V SERIES. 
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