REPRODUCTION IN UNICELLULAR ORGANISMS 



257 



overlooked. Thus we find a large number of the most diverse kinds 

 of cells, all of which serve for the maintenance of the body, in contrast 

 to the simply reproductive cells or germ-cells. These alone possess 

 the power of reproducing, under certain conditions, a new individual 

 of the same species. We can contrast with these germ-cells, which 

 serve, not for the maintenance of the individual, but only for that of 

 the species, all the other kinds of cells under the name of somatic or 

 body-cells. The problem which we have to solve now lies before us 

 in the question, How comes it that the germ-cell is able to bring forth 

 from itself all the other cells in definite sequence and arrangement, 

 and is thus able to build up the body of a new individual ? 



The similarity of this problem to that formulated in regard to 

 unicellular organisms is at once obvious, but it becomes still more 



sj: 





"^-n 



-sy^r^. 



I.;. 62. Pandorina morum ; after Pringsheim. I, A young colony, consisting of 16 cells. II, Another 

 my, whose cells have reproduced daughter- colonies ; all the cells uniformly alike. Ill, A young 

 :vox-colony ; sz, somatic cells ; ks, germ-cells. 



emphatic when we remember that the gulf between unicellular 

 organisms and the higher animals and plants is bridged over by 

 certain transition forms wdiich are of the greatest interest, especially 

 in relation to the problems of inheritance. 



Among the lower Algse there is a family, the Volvocinea?, in 

 which the differentiation of the many-celled body on the principle of 

 division of labour has just set in ; in some genera it has been actually 

 effected, though in the simplest way imaginable, and in others it has 

 not yet begun. Thus in the genus Pandorina the individual consists 

 of sixteen green cells, united into a ball (Fig. 63, I), each one exactly 

 like the other, and all functioning alike. They are all united into a 



I. E 



