LIFE OF ELIE METCHNIKOFF 109 



digestive cavity and only assumes the gastrula form 

 in its ulterior evolution. He also made tlie remark- 

 able discovery that, in the most primitive multi- 

 cellular animals, the endoderm is formed, not by means 

 of invagination, but by the migration of a number of 

 flagellated cells from one pole of the wall of the 

 blastula into the central cavity. These cells draw in 

 their flagellum, become amoeboid and mobile, multiply 

 by division, fill the cavity of the blastula, and become 

 capable of digesting. They originate the digestive 

 cells of the complete organism and give birth to the 

 mesoderm, which explains how the latter comes to 

 •contain a number of devouring cells even though 

 these do not constitute digestive organs properly 

 so called. Metchnikoff gave to that stage the name 

 of parenchymella, for the migrating cells constitute 

 the endoderm in the condition of a parenchyma. 



The invariable presence of this stage in the simplest 

 multicellular animals, the primitive amoeboid state 

 of the endodermic cells, cases of ulterior transforma- 

 tion of the parenchymella into the gastrula form in 

 certain animals, the absence of a differentiated 

 digestive cavity, — all that proved, according to 

 Metchnikoff , that the parenchymella is more primitive 

 than the gastrula, and is therefore entitled to be con- 

 sidered the prototype of multicellular beings. 



He saw a confirmation of this in the fact that 

 primitive adult animals also have no digestive cavity 

 but merely an intracellular digestion (sponges, tur- 

 bellaria). 



He concluded that the common ancestor of 

 multicellular beings was a being constituted by an 

 agglomeration of cells without a digestive cavity, but 

 endowed with intracellular digestion, like that of the 



