340 Metschnikoff on Germ-Layers. [April 
nies of flagellate Infusoria were transformed into primitive Meta- 
zoa explains very clearly the most important phenomena of 
metazoan development. * On this view the segmentation of the 
egg, and especially the more primitive total segmentation, has 
been derived from the division which the Flagellata undergoes 
in building up a colony. In like manner the fact that the cells 
of so many blastospheres are ciliated is probably due to inherit- 
ance from the Flagellata. This hypothesis forbids our homolo- 
gizing the mouth and other “ organs” of the Protozoa with the 
like parts of the Metazoa, but on the other hand enables us, as 
Bütschli (21) first pointed out, to comprehend the origin of sex- 
ual multiplication. As a fact most embryologists, Ray Lankester © 
and Balfour among others, have adopted this second hypothesis, 
and after a prolonged trial it has become a basis for further specu- 
lations. 
Having progressed this far, we should ask ourselves whether 
it is not possible, with the help of our present knowledge, to de- 
termine more or less exactly the nature of those Flagellate colo- 
nies from which the Metazoa are descended. Bütschli (22) be- 
lieves the Metazoa have had a double origin,—the Sponges he 
derives from colonies of the Choano-Flagellata, the rest of the 
Metazoa from colonies of true Flagellata. Aside from the fact that 
there is very little ground for such a venturesome assumption, 
we must remember that the two groups (of Flagellata) are not 
sharply separated, and that the collar, which constitutes the main 
point of difference, is in some cases entirely retracted. As to the 
relationship of the Sponges to the Ceelenterates, I shall have a 
word to say farther on. 
Whether the Flagellata from which the Metazoa are descended 
had a collar or not, they were certainly able to take in solid bits 
of food. This is to be inferred from the great prevalence of 
. intracellular digestion among the lower Metazoa. Taking into 
account this characteristic of the Metazoa-Flagellata, I cannot 
believe with Bütschli that the process of nutrition is not worth 
considering in connection with the question of the metazoan de- 
scent. Bütschli is of this mind “because the physiology of nu- 
trition varies exceedingly in the group of Flagellata, without 
to the morphology” (“ Remarks on the Gastræa Theory,” 
p. 417). : Drusi on the contrary, that the further differentia- 
tions undergone by the ancestral colonies were ai no means 
