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THE AMERICAN NATURALIST [Vol. L 



seems to me overwhelmingly convincing for regarding 

 them as truly— that is to say, genetically— homologous. 



Looking at the matter from another point of view, 

 namely, from the standpoint of the Metazoa, it is true that 

 in the groups of most complicated and highly organized 

 structure the cells often develop secondary connections or 

 fusions due to incomplete division, to such an extent that 

 in parts of the body the individuality of the primitively 

 distinct cells may be indicated only by the nuclei (as may 

 occur also in Protozoa, for example, in associated grega- 

 rines) ; but in all Metazoa certain of the cells retain per- 

 manently their complete independence and freedom of 

 movement and action. In the Metazoa possessing the sim- 

 plest and most primitive types or organization, such as 

 sponges and ccelenterates, the cells composing the body 

 show far greater independence of action, and in the course 

 of ontogeny entire groups of cells may alter their relative 

 positions in the body as the result of migrations performed 

 by individual cells ; while it is now well known that if the 

 adult sponge or hydroid be broken up completely into its 

 constituent cells, those cells can come together again and 

 build up, by their own individual activity, the regenerated 

 body of the organism. For these reasons it seems to me 

 impossible to regard the body-cells of the Metazoa other- 

 wise than as individual organisms complete in themselves, 

 primitively as independent as the individual Protozoon, 

 and in every way comparable to it. 



From the considerations summarized very briefly in the 

 two foregoing paragraphs and capable of much greater 

 amplification and elaboration, the view generally held that 

 the entire organism of a Protozoon is truly homologous 

 with a single body-cell of a Metazoon seems to me quite 

 unassailable, and to have gained in force greatly from re- 

 cent investigations upon both Protozoa and Metazoa. On 

 the other hand, any Protist, as an organism physiologic- 

 ally complete in itself, is clearly analogous to the entire 

 individual in the Metazoa— a comparison, however, which 

 leaves the question of genetic homology quite untouched. 



