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. ^ All the other cells are cap- 

 able of reproducing their like by that process of fission 

 which, as we have seen, is very prevalent among unicel- 

 lular 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 conju- 

 gating 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 



1 " This is very generally tlie 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 con- 

 ditions of the degeneracy of these parasites, for, frequent as her- 

 maphroditism is among plants and animals, self-fertilization is 

 extremely rare." — Hvohition of Sex, p. 71. 



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