Frenzel’s Mesozoon Salinella. 477 
augmented in the same way from among the endoderm cells, 
which were by this time multiplying to excess. But then it 
is absolutely impossible to see why precisely such a form of 
animal should exist, as one in which only so many ectoderm 
cells become amceboid as are necessary for the formation of 
the endoderm, in order not to leave any over in the shape of 
so-called mesoderm. The formation of the endoderm was 
indeed neither the object nor the cause, but merely the conse- 
quence of the immigration. As a matter of fact, such a bi- 
laminate animal as represented by the Gastrea can neither 
have existed in phylogeny nor be in existence to-day. 
That the Gastrula is nevertheless present in ontogeny is, 
as has already been mentioned, to be explained from the fact 
that the immediate incentive to the further development of 
the body from the Blastula, which in phylogeny was a more 
physiological process, dependent in a higher degree upon the . 
individuality of the cells, has here, in ontogeny, become a 
more mechanical necessity. The phylogenetic method is 
longer, and therefore in ontogeny it 1s adopted merely in the. 
case of very primitive forms (certain Porifera and Cnidaria), 
as opposed to which the more developed forms have gradually 
selected a shorter, because more mechanical, way to the same 
end. 
Now the various colonies of Flagellata, and especially 
Volvox, present us with the highest stage of colony-formation 
among unicellular creatures, nay even with the most primitive 
multicellular animal, which already appears to possess an 
integral individuality. At the same or a somewhat higher 
stage, but developed from other unicellular ancestors, we also 
find Salinella*, likewise an animal formed of an epithelial 
layer of cells, with an internal cavity. Now more and more 
cells—probably because they are weaker or stronger than 
their neighbours, and perhaps also because, owing to the axis 
of fission having been possibly somewhat oblique, they were 
situated more towards the interior—become continually 
forced from the epithelial position (if they are stronger than 
the rest they set themselves free), and passing into an 
amoeboid stage reach the inner cavity. It is possible that 
Trichoplax adherens corresponds to precisely this stage, in 
which, with the communication between the internal cavity 
and the exterior, the incentive to a secondary epithelial 
* That cells are found upon the ventral surface which are somewhat 
differently constituted to those upon the dorsal side, is in this case (as 
also in that of T’richoplax) the immediate result of the creeping, and no 
longer floating, mode of life, and would in itself indicate no higher posi- 
tion than that of Volvox, 
