246 COMPARATIVE ANATOMY 



for the products of cell-division to cohere together and form a 

 compound organism. In consequence of this coherence the 

 reproductive faculty of the ordinary cells is limited to the 

 increase of the bulk of the colony, and in order that the species 

 may be perpetuated, special individuals are produced to which 

 the function of reproducing new colonies is delegated. Thus 

 the union of cells into a stock or colony involves, it would seem 

 as a necessary consequence, a differentiation into two distinct 

 kinds of individuals. The one kind, comprising what we have 

 styled the ordinary individuals, are mainly nutritive in function 

 and may henceforth be called the vegetative, or, better, the 

 somatic cells. The other kind are almost exclusively repro- 

 ductive, and may henceforth be called the germ cells. It 

 should be noticed in passing that in Volvox and possibly also 

 in Zoothamnium, the germ cells are themselves divided into 

 separate kinds, asexual and sexual. The asexual germ cells 

 are the parthenogonidia of Volvox and the unfertilised macro- 

 gonidia (which, if they really exist, might be called by the same 

 name) of Zoothamnium. The sexual cells are the male and 

 female gonidia of Volvox, the microgametes and macrogonidia 

 of Zoothamnium. 



Now it has been shown that uni-cellular organisms in most 

 cases at any rate are incapable of propagating themselves in- 

 definitely by binary division. Sooner or later they must con- 

 jugate or they will wear out and perish. What is true of 

 uni-cellular organisms is true of the individual cells composing 

 a colony or a multi-cellular organism. After a more or less 

 prolonged exhibition of their vital activities they will degenerate 

 and perish if they do not receive a fresh impulse from conjuga- 

 tion. But the somatic cells are incapable of conjugation ; 

 that function is limited to the male and female germ cells. 

 Hence the union of cells to form a composite colony or in- 

 dividual, and their differentiation into somatic and germ cells 

 is fraught with the inevitable consequence of decay and death 

 for the somatic cells. 



It follows that the continued propagation of a race of multi- 

 cellular organisms, or cell colonies, in which there is a differen- 

 tiation into somatic and germ cells, is dependent upon the germ 

 cells, for they alone can conjugate and so rejuvenate the 

 enfeebled powers of the organism. They are the survivors, 

 whilst the somatic cells perish. 



