(1) The sex-chromosome in the male gametocytes is single and fails to divide with the others, but passes undivided to one pole. This may occur in the first reduction division (Orthoptera, Coleoptera, Diptera) or in 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 quadruple, while the other, say _Y_, is single. In all these cases there are two _X_ chromosomes in the oocytes (and somatic cells) of the female, and after reduction the female gametes or unfertilised ova are all alike, having a single _X_ chromosome or group. On fertilisation half the zygotes have _XX_ and half _XY_, whether _Y_ is absence of a sex-chromosome, or one of the other _Y_ forms above mentioned. The sex is thus determined by the male gamete, the _X_ chromosome united with that of the female gamete producing female individuals, while the _Y_ united with _X_ produces male individuals.