CHAPTER IV
One function which, in high animal organisms, is performed exclusively by one class of cells alone, the germ cells, is that of reproduction, that is, reproduction of the whole organism.[1] All the other cells are capable of reproducing their like by that process of fission which, as we have seen, is very prevalent among unicellular organisms. Thus muscle, nerve, or skin cells can severally produce their like, but no muscle, nerve, or skin cell can produce a cell of an unlike kind; still less can it produce a whole organism, in which there are many different kinds of cells arranged in definite relations to one another. Unlike the case in lower animals, such as sponges, then, this power among higher animals, such as vertebrates, is limited to the germ cells alone, each of which presents in addition the remarkable peculiarities unknown among any other of the cells, (1) of conjugating with another cell, and (2) of being incapable of reproducing other cells, like or unlike, except it first conjugates with another germ cell, and that germ cell not a near relation, not a descendant of the pair of germ cells from which the organism of which it forms a part
- ↑ "This is very generally the case, but it is not universal. 'Self' fertilization—that is, union of the eggs and sperms of the same organism—has been proved to occur in several trematodes, and to be almost universal in cestodes. This may be one of the conditions of the degeneracy of these parasites, for, frequent as hermaphroditism is among plants and animals, self-fertilization is extremely rare."—Evolution of Sex, p. 71.
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