Germ Plasm And Soma
( Originally Published 1923 )
5. Germ Plasm and Soma; Male and Female Bodies. -As stated on page 75, the essential fact of the sexual process, from a biological viewpoint, is the union of two germ cells or gametes. Among the lowest plants and animals these conjugating cells are formed out of cells that cannot be distinguished from the other cells of the body. But as organisms become more complex we find that early in the development of the embryo, certain of the cells retain their primitive capacity to divide indefinitely, while those that form the bulk of the body eventually lose this capacity. There is thus a separation between that portion of the developing protoplasm which becomes one particular individual and that portion which, though en-closed within the body of that individual, retains the ability to reproduce itself. It is from this latter portion that the gametes are formed, so that, generation after generation, there is a continuity of this reproductive material, which is called germ plasm, as distinguished from the soma or body plasm. Whatever befalls the individual, short of killing him, seems to leave the germ plasm unaffected. It is true that in certain cases defective individuals are born as a result, apparently, of some injury received by the germ plasm or by the developing embryo. But in such cases this injury is something that occurs apart from or in addition to any injury to the body of the parent. We may think of this germ plasm as being in a sense "immortal," for although it will die when the body which bears it dies or when it is injured by disease, it normally projects itself, through conjugation and development, into new bodies, while the body proper, apart from this germ plasm, cannot reproduce itself. We may therefore further think of the germ plasm as the stream of living matter which from time to time, under favorable conditions, throws off portions of itself or "buds" that develop into the individuals which we know.
The bearing of these germ cells involves, of course, an organ for eggs and an organ for sperms ; but among lower forms of life these are not very sharply differentiated from each other, and in certain species both may be borne by the same individual. As we come to higher forms we can distinguish between an ovary or egg-bearing gonad, and aspermary or sperm-bearing gonad, among plants as well as among animals. These organs may be considered as primary sexual structures, since they are part and parcel of the fact of sexual reproduction, the fact of producing gametes, that is. And the method by which eggs and sperms are discharged from these germ-bearing organs or gonads, at least among the lower forms, may be considered as primary sexual processes.
In some plants and simple animals the ovary, or egg-bearing gonad, and the spermary, or sperm-bearing gonad, simply burst open and discharge their contents into the water. There are no special organs. Or there may be a duct or passage-way through which the germ cells are discharged. In such organisms (for example, starfish, oyster) there is no outward difference between male and female, there is no difference in behavior between the two sexes. The two kinds of germ cells mature at about the same time, they are discharged at about the same time, and since the parent organisms would be swarming or crowded together in a given area, the sperms, swimming about, will find eggs to fertilize.
There are always many, many more sperms than there are eggs, so that most of them die or are destroyed without contributing to the perpetuation of the species. Very many eggs are destroyed without becoming fertilized, and among those that are fertilized very many cannot develop into mature individuals. The evolution of Iife has been in the direction of insuring a greater proportion of fertilizations, and especially of a greater proportion of survivals among the fertilized eggs. This has included the specialization of some individuals of the species as females and others as males.