342 Metschnikoff on Germ-Layers. [April 
mals with unequal segmentation. Even the formation of a blas- 
tosphere can take place without the occurrence of the typical 
first three divisions. Thus, in Volvox, all the divisions are 
meridional, the result being a plate-like embryo resembling 
Gonium. There is no segmentation cavity, and the blastoccele 
is formed by the gradual growth of the plate towards one pole 
(22, 23, 25). If we are to use the process of segmentation at all 
for genealogical purposes, the assumption adopted above seems 
inevitable. It is, moreover, not without the support of analo- 
gous cases in the organic world, as is learned from the divisions 
undergone by the Schizomycetes. Most of these forms divide 
transversely, but there are a few exceptions with longitudinal 
division, for example, a peculiarly branched species parasitic in 
Daphnia pulex, discovered and described by me as Dendrobac- 
terium oculatum. Besides such bacteria where there is but one 
kind of division, there are others where the cells divide in two 
meridional planes, as in the micrococci of gonorrhoea; and yet 
others like Sarcina, where the divisions follow the three dimen- 
sions of space and consequently agree with the total segmenta- 
tion of most Metazoa, and also with the assumed division of the 
hypothetical Metazoa-Flagellata. Since in the typical cases of 
total segmentation the segmentation cavity appears after the third 
division, and the embryo is very early transformed into a blas- 
tosphere, it is probable that the ancestors of the Metazoa swam 
about as blastosphere-like colonies, 
If we accept these peculiarities of the Metazoa-Flagellata as a 
basis for further speculations, we are enabled, it appears to me, 
to throw a certain light on the origin of the primitive organs. 
Embryology teaches us that the endoderm is formed in very dif- 
ferent ways among the Meduse. In recapitulating these various 
methods, I have first to state that the endoderm arises either 
at several points of the embryo or only at one point. In the 
first case the origin is multipolar, in the second unipolar (in the 
latter the pole is always at the hinder end of the larva). The 
multipolar type of formation appears either (a) as a multipolar 
immigration of the cells from the surface of the blastosphere 
into the interior ; (4) as a primary delamination by means of the 
transverse division of the cells of a blastosphere; (c) as a sec- 
ondary delamination following upon the formation of a morula; . 
or (d) as a mixed delamination, where the endoderm is in part 
