2o6 CONTINUITY OF THE GERM-PLASM AS THE [IV. 



cells, as explained in mj' earlier papers upon the same 

 subject. 



The phyletic origin of germ-cells obviousl}' coincides with the 

 differentiation of the first multicellular organisms by division of 

 labour \ If we desire to investigate the relation between germ- 

 cells and somatic cells, we must not only consider the highly 

 developed and strongly differentiated multicellular organisms, 

 but we must also turn our attention to those simpler forms in 

 which phj'letic transitions are represented. In addition to soli- 

 tary unicellular organisms, we know of others living in colonies 

 of which the constituent units or cells (each of them equivalent 

 to a unicellular organism) are morphologically and physiolo- 

 gically identical. Each unit feeds, moves, and under certain 

 circumstances is capable of reproducing itself, and of thus form- 

 ing a new colony by repeated division. The genus Pandorina 

 (Fig. I), belonging to the natural order l^olvociueae, represents 

 such ' homoplastid ' (Gotte) organisms. It forms a spherical 

 colon}' composed of ciliated cells, all of which are exactly alike : 

 they are embedded in a colourless gelatinous mass. Each cell 

 contains chlorophyll, and possesses a red eye-spot, and a 

 pulsating vacuole. These colonies are propagated by the 

 sexual and asexual (Fig. II) methods alternately, although in 

 the former case the conjugating swarm-cells cannot be dis- 

 tinguished with certainty as male or female. In both kinds of 

 reproduction, each cell in the colony acts as a reproductive cell ; 

 in fact, it behaves exactly like a unicellular organism. 



It is very interesting to find in another genus belonging to the 

 same natural order, that the transition from the homoplastid to 

 the heteroplastid condition, and the separation into somatic and 

 reproductive cells, have taken place. In Volvox (Fig. Ill) the 

 spherical colony consists of two kinds of cells, viz. of very nu- 

 merous small ciliated cells, and of a much smaller number of 

 large germ-cells without cilia. The latter alone possess the 

 power of producing a new colony, and this takes place by the 

 asexual and sexual methods alternatelj^ : in the latter a typical 

 fertilization of large egg-cells by small spermatozoa occurs. 



' Among unicellular organisms, encysted individuals are often called 

 germs. They sometimes difTer from the adult organism in their smaller 

 size and simpler structure [Grfgannidae , but they represent the same 

 morphological stage of individuality. 



