the second (many Hemiptera). But it is difficult to understand what is
meant by 'fails to divide.' In one of the reduction divisions all the
chromosomes divide as in ordinary or homotypic nucleus division, but in
the other the chromosomes simply separate into two equal groups without
division. If there are an odd number of chromosomes, 2_N_-1, in all the
gametocytes of the male, as stated in most accounts of the subject, then
if one chromosome fails to divide in the homotypic division, we shall have
2_N_-2 in one spermatocyte and 2_N_-1 in the other. Then when the
heterotypic division takes place and the number of chromosomes is halved,
we shall have two spermatocytes with _N_-1 chromosomes from one of the
first spermatocytes and one with _N_ and one with _N_-1 from the other.
Thus there will be three spermatozoa with _N_-1 chromosomes and one with
_N_ chromosomes, whereas we are supposed to find equal numbers with _N_
and _N_-1 chromosomes. It is evident that what Dr. Wilson means is
that the sex-chromosome is unpaired, and that although it divides
like the others in the homotypic division, in the heterotypic division
it has no mate and so passes with half the number of chromosomes to one
pole of the division spindle, while the other group of chromosomes has
no sex-chromosome. Examples of this are the genera _Pyrrhocoris_ and
_Protenor_ (Hemiptera) _Brachystola_ and many other Acrididae, _Anasa,
Euthoetha, Narnia, Anax_. In a second class of cases the sex-chromosome is
double, consisting of two components which pass together to one pole.
Examples of this are _Syromaster, Phylloxera, Agalena_. In a third class
the sex-chromosome is accompanied by a fellow which is usually smaller,
and the two separate at the differential division. The sizes of the two
differ in different degrees, from cases as in many Coleoptera and Diptera
in which the smaller chromosome is very minute, to those (_Benacus,
Mineus_) in which it is almost as large as its fellow, and others
(_Nezara, Oncopeltus_) in which the two are equal in size. Again, there
are cases in which one sex-chromosome, say _X_, is double, triple, or even