LIFE OF ELIE METCHNIKOFF 277 



formation of the simple to the complex and to see the 

 origin and development of all the constituent parts 

 of the organism. Moreover, the embryo is exempt 

 from secondary complications, due to the multiple 

 external conditions of post-embryonic life. 



Metchnikoff was able to establish, from embryo- 

 logical data, that the development of lower animals 

 takes place according to the same plan and under 

 the same laws as that of higher animals. In all of 

 them, the segmentation of the egg is followed by the 

 formation of embryonic layers, of which each gives 

 birth to cells and to definite organs. Superior forms 

 repeat, in their embryonic life, the evolution cycle of 

 inferior forms. 1 



This common plan in the embryology of all 

 animals established their genealogical continuity and 

 strengthened the Darwinian theory. 



Metchnikofi's studies, carried out on the various 

 groups of animals, contributed towards the founda- 

 tion of comparative embryology. Owing to the 

 comparative method, he had made himself familiar 

 not only with the morphological and functional con- 

 tinuity of divers organisms, but also with that of 

 their constituting cells ; a comparison between the 

 latter and unicellular beings was inevitable. That is 

 why, having ascertained that the mobile cells of the 

 lower Metazoa absorbed foreign bodies by inclusion, 

 he naturally concluded that that phenomenon was 

 similar to digestion in unicellular beings. 



Having established the fact of intracellular diges- 

 tion in lower animals, he extended it to certain cells 



1 Thus the parenchymella, phagocytdla, and gastrula stages correspond 

 in the embryo with the adult form of certain very primitive Metazoa and 

 even to a colony of unicellular animals. 



