LIFE OF ELIE METCHNIKOFF 51 



cellular beings through division ; only, those segments 

 of the ovum do not separate but constitute a whole 

 under the aspect of a hollow sphere, called a blastula, 

 which is the first manifestation of a multicellular 

 being. This blastula is formed of superposed layers, 

 each of which gives birth to specialised organs in the 

 embryo. The outside layer, or ectoderm, produces 

 teguments and the nervous system ; the internal layer, 

 or endoderm, gives birth to endothelial cells, the digestive 

 and internal organs ; between those two layers comes 

 a third, intermediary layer, the mesoderm, from which 

 the skeleton is developed and also the muscle and 

 blood tissues. 



The evolution of these layers in Vertebrates was 

 well known, but very little so in Invertebrates, though 

 it is only through the development of inferior forms 

 that the origin and general evolution of living beings 

 can be elucidated. That is why, during many years, 

 the principal theme of Metchnikoff's researches was 

 the comparative study of the embryonic layers of 

 inferior animals and the ulterior fate of their con- 

 stituting elements. By following this train of thought, 

 he was able to demonstrate that the development of 

 lower animals takes place on the same plan and 

 follows the same laws as that of higher animals ; 

 thus, that there is a real communion between all 

 living beings, which is the concrete confirmation of 

 the theory of evolution. 



By their work, Kovalevsky and Metchnikoff con- 

 tributed to the foundation of Comparative Embryo- 

 logy. The comparative study of cells produced from 

 the divers embryonic layers, and observations on the 

 ulterior development of the functions of those cells, 

 gradually led Metchnikoff to his theory of phagocytes 



