THE REPRODUCTIVE CYCLE 217 



In Type 6 the conditions are shown for a simple many-celled 

 organism like the animal hydra, or one of the lower plants. This 

 differs from Type 5 only in the number of kinds of body cells. 

 In Type 5 there is but one kind of body cell. In Type 6 there are 

 several, as found in simple metazoa (Fig. 121, p. 250). In 

 the higher plants and animals the difference consists only in the 

 greater number of kinds of body cells and their organization as 

 tissues, organs, and systems; until in an animal like a vertebrate 

 or one of the flowering plants there are innumerable body cells of 

 many sorts. Functional differences are, of course, correlated 

 with these structural differences. The cell in the multicellular 

 bod}^ is a unit that is largely dependent upon the life cf the 

 organism as a v/hole.^ We may thus distinguish between body 

 and germ cells in many-celled animals and compare the germ cells 

 with cells of protozoa, as the table shows (Fig. 110), without 

 supposing that these two types of cells are always sharph' dis- 

 tinguishable in the manner implied by extreme theories of the 

 independence of the germ cells or germplasm, as they may be col- 

 lectively termed in contrast with the somatoplasm which is the 

 whole mass of body cells. 



The foregoing comparisons of cell cycles in many-celled and 

 single-celled organisms are, of course, made from the viewpoint 

 of the Cell Theor3\ According to the Organismal Theory {cf. 

 p. 194), the homologies of cells and cell acti\aties that are here 

 indicated should not receive so much emphasis, since the organism 

 must be considered as a whole and not as a mere colony of cells 

 differentiated for various functions. The existence of protozoa 

 (Fig. 102, p. 194) so highly specialized that they are more complex 

 than any other cells further emphasizes the importance of the 

 individual as compared with the cell. As already stated, a more 

 reasonable position would seem to be the recognition of organisms 



^ In plants and in animals that are capable of regenerating the whole organ- 

 ism from a part {cf. p. 273), the body cells are dependent upon a certain mass 

 of the whole, which may be capable of regeneration, rather than upon the 

 entire organism. In tissue cultures, small masses of body cells may be culti- 

 vated for long periods, if not indefiniteh'. The cells of some sponges may be 

 completely dissociated by squeezing through bolting cloth and will even then 

 form new individuals. But in none of these cases have the single body cells 

 been showTi to be capable of reproducing the whole as do single germ cells. 

 There is, however, no sound theoretical reason why such cells might not thus 

 reproduce the whole if they could be placed under proper conditions. 



