Sex as an Adaptation 433 
this time, and the two nuclei, uniting into a single one, fur- 
nish the total number of chromosomes characteristic of the 
animal that develops from the egg. At first the chromosomes 
that are brought in by the spermatozoon lie at one side of 
the fused nucleus, and those from the egg itself at the other 
side. This arrangement appears, however, in some cases 
at least, to be lost later. At every division of the nucleus, 
each chromosome divides and sends a half to each of the 
daughter-nuclei. Thus every cell in the body contains as 
many paternal as maternal chromosomes. This statement 
also applies.to the first cells that go into the reproductive 
organs, some of which become the mother-cells of the germ- 
cells. Later, however, in the history of the germ-cells, — 
just before the maturation divisions, —these chromosomes 
are supposed to unite in pairs, end to end, as explained 
above, to give the reduced number. Later there follows 
the separation of these paired chromosomes at one of the 
two maturation divisions. If at this time all the paternal 
chromosomes should pass to one pole, and all the maternal 
to the other, the germ-cell ceases to be mixed, and becomes 
purely paternal or maternal. If this ever occurs, the problem 
of heredity may become simplified, and even the question of 
sex may be indirectly involved; but it has not been established 
that, when the reduced number of chromosomes is formed, 
there is a strict union between the paternal and maternal chro- 
mosomes, and if not, the subsequent separation is probably 
not along these lines. If, however, the chromosomes contain 
different qualities, as Boveri believes, there may be two kinds 
of eggs, and two kinds of spermatozoa in regard to each 
particular character. It is this last assumption only that is 
made in Mendel’s theory of the purity of the germ-cells. 
Several attempts have been made at different times to 
connect the facts in regard to the extrusion of the polar 
bodies with those involved in the determination of sex. 
Minot suggested, in 1877, that the egg ejects by means of the 
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