When the spermatozoon enters the egg it brings into the egg as many new chromosomes as the egg itself possesses at this time, and the two nuclei, uniting into a single one, furnish 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 chromosomes, 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.