336 THE EVOLUTION OF THE METAZOA 



ontogeny, how is it then possible that this situation is not 

 preserved and why does the internal structure return into 

 the plasmodial state? To this we can remark that in all proba- 

 bility the physiological factors of the evolution as well as the 

 inherited properties could be responsible for such a develop- 

 ment. Moreover, we can call attention to the fact that even 

 in Metaphyta and in (as we have just mentioned) Spongiae 

 w hich both had certainly evolved from some colonies of cells, 

 we can observe during a certain stage an incUnation to form 

 syncytia. This stage is again followed by a period of cel- 

 lularization. 



The second and even more important indication can be 

 found in the long list of similarities that can be observed in 

 the Infusoria on the one hand, and in the acoelous Turbellaria 

 on the other. These similarities make it probable that 

 the Eumetazoa had evolved from the polykaryonic Infusoria. 

 We can therefore conclude that initially the postgamous divi- 

 sions were limited to the karyons only, that the youngest 

 embryos had a plasmodial character so that no recapitulations 

 take place any longer in the early development. 



Zawarzin (1945-1947) was not influenced, by the facts of 

 cleavages, against the colonial theory. He placed the separa- 

 tion of the monocellular and polycellular animals far back 

 into the remote geologic past, to a period when the separation 

 of the animal and plant lines of evolution had taken place. 

 The Metazoa had evolved, according to his interpretation, 

 without any differentiation parallel to the Protozoa from the 

 polyenergid plasmatic masses. This is a very radical concept 

 which lacks probability because it presupposes that the sexual 

 phase evolved in both lines (thus parallel to the plantal line) 

 at least twice independently from each other and that it had 

 led in spite of this to similar conditions. Neither does this 

 concept explain the numerous similarities that exist between 

 the Infusoria and the Turbellaria. How can we then interpret 

 the not rare cases of the polykaryonic Protozoa? 



