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 



2 F 



