108 Dli. ELIAS METSOHNIKOFF. 



cells^ which, like all the others of the colony, had the power of 

 obtaining and ingesting food, travelled from various parts of 

 the periphery of the blastosphere to the interior, losing their 

 columnar character, and becoming amoeboid. These amoeboid 

 cells I regard as the rudiment of a parenchymatous endo- 

 mesoderm, which afterwards diflferentiated into true endoderm 

 and other structures.^ 



The discovery of ectoderm cells, which retain the power of 

 ingesting food, in various groups of Ccelenterates, seems to 

 me totally incompatible with an early differentiation of the 

 blastula cells, such as is assumed by Balfour, and rather goes 

 to show that at the stage in question the whole ectoderm 

 habitually performed those functions which are now limited to 

 the endoderm. It is very difficult to suppose that so funda- 

 mental a property as the nutrient power of a Monad cell should 

 have disappeared in so short a time as that occupied by the 

 differentiation of the two halves of the ancestral Sycandra. 



Further, I am aware of no reason, embryological or other- 

 wise, which prevents our assuming that the germinal layers 

 were differentiated at a time when all the cells of the organism 

 retained the power of taking up food. Later on, this power 

 was restricted to the parenchyma cells, as in Sponges, where 

 the food is taken in by a parenchymatous mesoderm, which 

 cannot be clearly distinguished from the endoderm, as even 

 in the adult cells are continually passing from one layer to 

 the other. Only later in phylogeny was a sharp distinction 

 established between the two divisions of the primitive paren- 

 chyma, or, as it may be called, phagocytoblast. But in the 



one half of the individuals of wliich have become dififerentiated into nutritive 

 forms, the other half into locomotor and respiratory forms. . . , That 

 the passage from the Protozoa to the Metazoa may have been effected by such 

 a differentiation is not improbable on a priori grounds." 'Comparative 

 Embriology,' i, p. 122. [This is quoted from the English edition. In the 

 German translation, quoted by Metschnikoff, Balfour is made to assume the 

 passage of all Metazoa 'through a form like that of the Sycandra larva, an 

 opinion which he certainly never expressed. — Translator.] 



1 " Spougeologische Studien," ' Zeitsch. f. vp. Zoologie,' 1879, Bd. xxxii, 

 p. 375, et seq. 



