III.] LIFE AND DEATir. 1 49 



be possible to regulate the relations of the single cells of the 

 colony to each other by fixing the number of cells within narrow 

 limits. During the development of Magospliaera—one of the 

 Homoplastides— the cells arrange themselves in the form of 

 a hollow sphere, lying in a gelatinous envelope. But the fact 

 that reproduction does not follow the simple unvarying rhythm 

 of unicellular organisms is of more importance; for a rhythm 

 of a higher order appears, in which each cell of the colony 

 separates from its neighbours, when it has reached a certain 

 size, and proceeds by very rapid successive divisions to give 

 rise to a certain number of parts which arrange themselves as 

 a new colony. The number of divisions is controlled by the 

 number of cells to which the colony is limited, and at first this 

 number may have been very small. With the introduction of 

 this secondary higher rhythm during reproduction, the first 

 germ of the Pol3^plastides became evident ; for then each pro- 

 cess of fission was not, as in unicellular organisms, equivalent 

 to all the others ; for in a colony of ten cells the first fission 

 differs from the second, third, or tenth, both in the size of the 

 products of division and also in remoteness from the end of 

 the process. This secondary fission is what we know as 

 segmentation. 



It seems to me of little importance whether the first process 

 of segmentation takes place in the water or within a cyst, 

 although it is quite possible that the necessity for some pro- 

 tective structure appeared at a very early period, in order to 

 shield the segmenting cell from danger. 



It is impossible to accept Gotte's conception of the germ 

 (Keim), and at this point the question arises as to its true 

 meaning. I should propose to include under this term every 

 cell, cytode, or group of cells which, while not possessing the 

 structure of the mature individual of the species, possesses the 

 power of developing into it under certain circumstances. The 

 emphasis is now laid upon the expression development, which is 

 something opposed to simple growth, without change of form. 

 A cell which becomes a complete individual by growth alone is 

 not a germ but an individual, although a very small one. For 

 example, the small encapsuled Heliozoon, which arises as the 

 product of multiple fission, is not a germ in our sense of the 

 word. It is an individual, provided with all the characteristic 



