270 RECENT CYTOLOGY 
particles of the respective characters concerned to 
remain in company during the process by which the 
other allelomorphs are being reassorted between the 
chromosomes. Of this process of coupling the cytolo- 
gists have not yet been able to observe any visible 
indication in the behaviour of the chromosomes, any 
more than they can really see the redistribution of the 
supposed factors carried by the chromomeres. But 
apart from this it must be allowed that the facts of 
experiment and of microscopic observation fit in with 
one another in a remarkable way, and that the Men- 
delian theory throws considerable light on the minute 
features of cell anatomy. 
The possibility still remains that in certain cases 
particular characters may be associated with par- 
ticular chromosomes as a whole, and we shall next 
proceed to describe what actually seems to be an 
example of this sort. 
The case we have to describe is directly concerned 
with one of the most interesting and elusive of bio- 
logical problems—namely, the problem of the heredity 
of sex. Until recently opinion has been largely 
dominated by the view that sex-production is in 
general controlled by the influence of external con- 
ditions. A large number of the earlier researches, 
and some of the later ones, have, in fact, seemed to 
show that sex is thus determined. The most recent 
knowledge appears, however, to point conclusively to 
the belief that sex is already determined in the fer- 
tilized ovum. The fact that, so far as the evidence 
