106 LIFE OF ELIE METCHNIKOFF 



adult existence, preceded by a long period in the 

 larval state. Thousands of these diaphanous, ephe- 

 meral insects swarm above the water in a compact 

 cloud ; now and then, dead Ephemeridae fall like 

 snow-flakes, and that is the final and tragic completion 

 of the nuptial flight. Metchnikoff wished to unveil the 

 mechanism of this sudden death, evidently due to 

 a physiological cause ; but he obtained no definite 

 results either that year or the following, when he 

 continued his observations in the Caucasus. He 

 realised that the life of these insects was too short to 

 allow him to solve the problems which interested him, 

 and, his eyes now being cured, he went back to his 

 studies on the origin of multicellular beings or metazoa. 



He studied the development of inferior sponges 

 and ascertained that they possess the three embry- 

 onic layers which correspond to those of other animal 

 types, but that these layers have not the same degree 

 of independence or differentiation. He found that 

 in certain inferior sponges the mesoderm develops 

 before the endoderm and gives birth to it. These two 

 layers, born one from the other, manifest common 

 primordial characters. Therefore he was in no wise 

 surprised to discover that, in these inferior sponges, 

 the amoeboid and mobile cells of the mesoderm fulfil 

 digestive functions equally with, and even more than 

 those of the endoderm; in fact, with primitive beings, 

 functional characters are not more strictly delimit- 

 ated than morphological characters. It is only a 

 more advanced differentiation which separates them. 



He connected these new facts with that which he 

 had observed in 1865 in one of the lower worms, the 

 earth planarian Geodesmus bilineatus. This worm is 

 actually without a digestive cavity, for the latter is 



