304 THE EVOLUTION OF THE METAZOA 



The third addition to my concept which was made by 

 Hanson regards "The transition from ciHated protozoan to 

 acoel." It consists in the suggestion that cellularization did 

 not take place earher than in the Turbellaria and that it can 

 therefore not be considered as the decisive step in the evolution 

 of Metazoa proper. The essential difference between the 

 Infusoria and the Acoela, and thus the decisive step in the 

 evolution of Metazoa, had been, according to Hanson, the 

 fact that in the Acoela the karyons had become morphological- 

 ly complex, in comparison with the Infusoria where they are 

 physiologically "compound." A progressive organization had 

 become possible because of this step. The variously placed 

 karyons had become heterogeneous, and this heterogeneity 

 w^as increased even more by a subsequent formation of cell 

 membranes, by a visible cellularization or compartment- 

 alization. 



Hanson is certainly right w^hen he attaches great importance 

 to the differentiation of the somatic karyons. This, however, 

 is not something entirely new because I too have thought 

 of such a differentiation in the form of a regular distribution 

 of karyons in all the three body layers. Neither have I at- 

 tributed any primary role to the cellularization because it does 

 not appear earlier than in the already "finished" yet primitive 

 Acoela, and even then only partly. Moreover, I believe that 

 it is a mistake to attribute a leading role to a single peculiarity 

 in the creation of the Acoela ("crucial point" or "key step," 

 according to Hanson). It seems to me much more probable 

 that they had emerged because of a convergence of several 

 important characteristics and moments, among which w^e must 

 mention above all the internalization of the sexual phase 

 combined with hermaphroditism. 



After this discussion of Hanson's study on the primitive 

 Infusoria I wish now to refer to the important work by I. B. 

 Rajkov (1957). Rajkov discusses the problem of the origin 

 of the bikaryonic state of Infusoria using in his study the 

 discoveries that have been recently made in connection with 



