REPRODUCTION IN UNICELLULAR ORGANISMS 259 



reproduce the whole colony but only cells like themselves, that is, 

 other somatic cells. In Volvox the maintenance of the species, the 

 production of a daughter-colony, is the function of the second and 

 larger kind of cells, the reproductive cells, which are contained in the 

 interior (filled with a watery fluid) of the gelatinous sphere. They 

 possess no flagella (kz), and so take no share in the swimming 

 movements of the somatic cells. For the present we need not allude 

 to the fact that there are several kinds of these cells, and need only 

 state that the simplest among them, the so-called ' Parthenogonidia,' 

 after they have reached a considerable size, begin a process of division 

 which results in the formation of a daughter-colony. Usually there 

 are several of these large reproductive cells in a Volvox colony, and 

 as soon as these have developed into a similar number of daughter- 

 colonies they burst out through a rupture in the now flaccid jelly of 

 the maternal sphere and begin to lead an independent life. The 

 mother-sphere, which now consists only of somatic cells, is unable to 

 produce new reproductive cells ; it gradually loses its spherical form, 

 sinks to the ground, and dies. 



In Volvox we have, for the first time, a cell-colony in which a 

 distinction has been established between body or somatic cells and 

 reproductive or germ-cells. In contrast to Pandorina, a large 

 number, indeed the majority of the cells of the colony, have lost the 

 power of reproducing the whole by division, and only the few 

 reproductive cells possess this, while they, in turn, have lost other 

 functions, notably that of locomotion. Their power of reproducing 

 the whole, that is to say, their hereditary capacity, gives them a 

 greater theoretical interest than the cells of Pandorina, for the latter 

 require only to produce others like themselves, because there is only 

 one kind of cell in the colony, while in Volvox the reproductive cell 

 can not only produce others like itself, by division, but can 

 produce the body-cells as well. The problem is quite analogous to 

 the one which we have had to face in regard to the unicellular 

 animals of complex structure, the Infusorians. The question. How 

 can the part of the trumpet-animalcule which is mouthless develop 

 from itself a new mouth and ciliated apparatus ? here transforms itself 

 into the question. How can a cell by division give rise not only to 

 others like itself, but also to the body-cells, which are of quite 

 different structure 1 This is, in its simplest form, the fundamental 

 problem of all reproduction through germ-cells, to which we must 

 now pass on. But first a short digression. 



We have already noted that unicellular organisms multiply by 

 division, and originally, as well as in the great majority of cases 



R 2 



