LIFE OF ELIB METCHNIKOFF 107 



entirely filled by parenchymatous cells inside which 

 digestion takes place. 



By their primitive structure, lower sponges and 

 worms come near the higher Infusoria, to which they 

 are even more closely related by this intercellular 

 digestion which is common to them. 



This led Metchnikoff to ask himself whether this 

 was not, generally speaking, the primitive mode of 

 digestion. He carried out numerous researches on 

 this point during the following years, and found the 

 same intercellular digestion in other lower worms, 

 such as the Mesostoma and aquatic planarians, and 

 afterwards in some lower Coelentera and some Echino- 

 derma. He was thus enabled to establish definitely 

 that the primitive mode of digestion was really inter- 

 cellular, for the lower multicellular animals either do 

 not possess any digestive cavity or else their digestive 

 cavity develops late, as for instance with lower jelly- 

 fish or with hydropolypi. Even when the cavity is 

 developed in these inferior animals, the digestive 

 functions are fulfilled by the mesodermic cells. 



The question as to what are the ancestral forms of 

 multicellular animals cannot be solved through direct 

 observation, for there is a lacuna between them and 

 unicellular beings, a lacuna which is due to the dis- 

 appearance of intermediary forms. It can only be 

 filled by hypotheses, based upon the embryology of 

 those animals which, in their embryonic development, 

 repeat the inferior forms from which they are derived, 

 thus reflecting the general evolution of living beings. 

 It was therefore to the embryology of lower multi- 

 cellular beings that Metchnikoff turned, in order to 

 endeavour to reconstitute their origin and to show the 

 link between them and unicellular beings. 



