202 THE CONTINUITY OF THE GERM-PLASM AS THE 



a colony of polypes, the germ-cells are produced by the later genera- 

 tions, and not by the founder of the colony which was developed 

 from an egg. This is also true of the colonies of Siphonophora, 

 and the germ-cells appear to arise very late in certain instances of 

 protracted metamorphosis (Echinodermata), but on the other hand, 

 they arise during the embryonic development of other forms (In- 

 secta) which also undergo metamorphosis. It is obvious that the 

 phyletic development of colonies or stocks must have succeeded 

 that of single individuals, and that the formation of germ-cells in 

 the latter must therefore represent the original method. Thus 

 the germ-cells originally arose at the beginning of ontogeny and 

 not at its close, when the somatic cells are formed. 



This statement is especially supported by the history of cer- 

 tain lower plants, or at any rate chlorophyll-containing organisms, 

 and I think that these forms supply an admirable illustration of 

 my theory as to the phyletic origin of germ-cells, as explained in 

 my earlier papers upon the same subject. 



The phyletic origin of germ-cells obviously coincides with the 

 differentiation of the first multicellular organisms by division of 

 labour l . 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 

 phyletic transitions are represented. In addition to solitary 

 unicellular organisms, we know of others living in colonies of which 

 the constituent units or cells (each of them equivalent to a uni- 

 cellular organism) are morphologically and physiologically identical. 

 Each unit feeds, moves, and under certain circumstances is capable 

 of reproducing itself, and of thus forming a new colony by repeated 

 division. The genus Pandorina (Fig. I), belonging to the natural 

 order Volvocineae, represents such 'homoplastid' (Gb'tte) organisms. 

 It forms a spherical colony 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 



1 Among unicellular organisms, encysted individuals are often called germs. 

 They sometimes differ from the adult organism in their smaller size and simpler 

 structure (Gregarinidae), but they represent the same morphological stage of in- 

 dividuality. 



