1887] Metschnikoff on Germ-Layers. 341 
independent of the method of nutrition. While in some colo- 
nies vegetable pigments were found to assist the process of as- 
similation, others, retaining the method of animal nutrition, gave 
rise to individuals whose special function it was to seize and 
digest food. That this conclusion is not purely deductive is 
gathered from a comparison of such Flagellate colonies as the 
Volvocinez and Protospongia. 
It would be much to the interest of further deductions if we 
possessed some actual knowledge of the development of these 
hypothetical colonies. That they must have been propagated 
sexually is clear from the multiplication of Volvox. Indeed, the 
existence of sexual multiplication is a strong argument in favor 
of the descent of the Metazoa from the Flagellata. As regards 
a sexual multiplication, the existing Flagellata divide and ex- 
hibit some variety in their division. The true Flagellata divide 
for the most part longitudinally, but transverse division occurs 
in some species, for example in Phalansterium consociatum ac- 
cording to Cienkowsky (24), and in Ph. digitatum according to 
Stein (25). In the Choano-Flagellata, also, both kinds of division 
have been observed, even in closely-related forms. Thus, ac- 
cording to Kent, Salpinggca campanula suffers longitudinal 
division (20), while all the other species of this genus divide 
transversely. “The simultaneous occurrence of longitudinal 
and transverse division in one and the same form has, however, 
been hitherto established only for certain Chlamydomonadine 
(23). Since in animals that ‘build up colonies the division of the 
individual plays an important part in determining the shape of the 
stock, it is important to learn how the hypothetical Metazoa-Flag- 
ellata behaved in this respect. Let us recall the generalization 
made in the second chapter, that the first three planes of segmen- 
tation lie in the three dimensions of space. We found this to be 
true for Medusz that suffer totally different developments (for 
hypogenetic as well as metagenetic Medusz, and regardless of 
the various ways of forming the endoderm, etc.) ; and it holds for 
animals in general, however different they may be, that undergo 
total segmentation. We are therefore justified in assuming that 
the same kind of division prevailed among the ancestors of the 
Metazoa. There is the more reason for this assumption in view 
of the many ways in which it is possible for an embryo to be 
built up, of which an idea may be obtained from plants and ani- 
