FrenzeVs Mesozoon Salinella. 477 



augmented in tlic same way from among the entlodcrm 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 amoeboid 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 Gostnea 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 is adopted merely in the 

 case of very primitive forms (certain Porifera and Cnidaria), 

 as opposed to which the more develo]ied 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 cidhcerens 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 Trichoplax) 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. 



