Page:Popular Science Monthly Volume 15.djvu/161

Rh stages in the developmental process are sufficiently alike in different animals to admit of a comparison between them, the stage at which impregnation takes place is not fixed, but variable. In some cases the ovarian egg remains without change until it is impregnated; and the first step in the developmental process, the disappearance of the germinative vesicle, is the immediate result of the union of the spermatozoa with the ovum. In other cases the germinative vesicle disappears, and the egg then remains inactive until it is impregnated; and this is followed at once by segmentation. In still other cases segmentation takes place without impregnation. Other eggs develop still further; and, finally, there are many animals whose unfertilized eggs not only commence, but complete the developmental process, and give rise to adults which may in turn produce young in the same way: and this may go on indefinitely, without the intervention of a male. The queen bee is able to lay fertilized or unfertilized eggs at will, and they are equally alive and capable of development.

These facts show conclusively that the essential function of the male element is not the vitalization of the germ.

Turning now to another aspect of our subject, we find that among plants, and among all the lower and simpler groups of animals, new individuals are produced by the various forms of asexual generation, as well as sexually., In certain animals, such as the tunicates, this form of generation is highly specialized, and the stolon [sic] from which new individuals are budded off is a highly complex structure, which contains cells or tissues derived from all the essential organs and systems of the parent, and from these the corresponding organs and systems of the new individual are derived. As a rule, however, the process of budding is very simple: a mass of unspecialized cells at some definite point upon the body of the parent, animal or plant, becoming converted into a new individual, instead of contributing to the further growth of the old. Among the lower animals, such as the hydroids and sponges, the process is still more simple, and cells may become converted into a bud at almost any point upon the body of the parent. That the process of reproduction by budding is not in any way absolutely distinguished from the process of ordinary growth by cell-multiplication, is shown by the fact that an accident may determine which of these processes is to result from the activity of a given cell.

Comparison shows that there is, on the one hand, no essential distinction between ordinary growth and reproduction by budding, and, on the other hand, none except the necessity for impregnation to distinguish asexual from sexual reproduction. All these processes are fundamentally processes of cell-multiplication. As none of the animals with which we are thoroughly familiar reproduce asexually, we are unable to make any very exact comparison of the results of the two processes of reproduction in animals; but among plants such