Page:Popular Science Monthly Volume 49.djvu/635

Rh incidentally, to vegetative or non-sexual reproduction, which presents no further features of interest, since it differs from ordinary growth only in that the product of this form of growth does not usually remain attached to, and form a part of, the parent individual. On the contrary, it becomes separated, in most cases, from the parent, and sets out as a new individual. But in sexual reproduction we meet with a new complication. The phenomenon of sexual union, which occurs, at least occasionally, in an enormous majority of known organisms, and in very many must always precede reproduction, is essentially a fusion of two cells. And, since the male cell often consists of little more than a nucleus, it may perhaps be reduced, in its final expression, to a fusion of two nuclei. Now it is observed that the number of chromosomes in a dividing nucleus of a given species of plant or animal is approximately constant, and in the sexual nuclei quite so. After a male sexual nucleus containing, for example, twelve chromosomes has united with a female nucleus containing the same number, the fertilization nucleus thus produced proceeds to divide, and is seen to contain twenty-four chromosomes, or as many as were brought to it by both parent nuclei. And this number is found to persist without great variation in the nuclei of the new organism developed from the fertilized cell by successive divisions. It is plain that if the sexual elements produced from organisms of this generation contained twenty-four chromosomes each, those of their sexually produced offspring would have forty-eight each, and the point would soon be reached by successive doublings at which the capacity of the nucleus would be far overtaxed by the number of chromosomes. But this difficulty is avoided, in the plants and animals thus far investigated, by an abrupt reduction to one half the number usual in the organism, of the chromosomes of the nuclei of certain cells which are to give rise to the sexual cells. This reduced number remains constant in all the descendants of the nuclei in which it first appears, until the definitive sexual cells are formed. Then the fusion of two nuclei, each with the half number of chromosomes, restores to the resulting organism the typical number. This reduction has been spoken of as abrupt; and it could not well be more so. A nucleus in which it occurs receives from its mother nucleus, let us say, twenty-four chromosomes which fuse together to form its nuclear network. When, after a period of rest, this nucleus proceeds to divide, it develops from its network but twelve chromosomes, and therefore furnishes but twelve to each of its daughter nuclei. What has become of the other twelve no one can say, because nothing is known of the exact relations that exist between the individual chromosome of the dividing nucleus and any part of the network of the resting stage.