XXVIU PKOCEEDINGS OF THE 



same sexual differeatiation and reproduction are found in unicellular 

 plants. 



It may now, then, be regarded as proved that the process by 

 which the body of the ciliate Infusorium attains a certain degree 

 of differentiation is repeated not only in other unicellular organisms 

 but in many parenchyma-cells both of plants and animals. The dif- 

 ference, as Haeckel with much force points out, between the differen- 

 tiation-process of these parenchyma-cells and that of the Infusorium- 

 body consists in the fact that in the parenchyma-cells the differentia- 

 tion is a one-sided one, conditioned by the division of labour in the 

 organism of which they form the constituents ; while in the Infu- 

 sorium it is a many-sided one related to all the different directions 

 in which cell-life manifests itself, and resting on a physiological 

 division of labour among the " plastidules " or protoplasm-molecules. 

 In other words, the differentiation-processes which in multicellular 

 organisms are found distributed among different cells, are united in 

 the single cell of the ciliate Infusorium, thus leading to the forma- 

 tion of an animal very perfect in a physiological point of view, but 

 which morphologically does not pass the limit of a simple cell. 



In some rarer cases the Infusorium-body is found to enclose two or 

 more nuclei ; and Haeckel admits that such Infusoria must strictly 

 be regarded as multicellular, since the nucleus in itself alone deter- 

 mines the individuality of the cell ; but these exceptional cases have 

 no significance for the main conception of the infusorial organism. 

 The multiplication of the nucleus exerts almost no influence on the 

 rest of the organization ; and such "multicellular Ciliata " are to be 

 compared with the colony-building forms of the Acinetse, Gregarinse, 

 Flagellatae, and other undoubtedly unicellular organisms. 



In conclusion, Haeckel considers the systematic position of the 

 Infusoria. That they are genuine Protozoa, having no direct 

 relation to either the Coelenterata or the Worms, must be now 

 admitted. To this result we are led in the most convincing way by 

 all that we know of their development. In all the animal types 

 which stand above the Protozoa the multicellular organism is de- 

 veloped out of the simple egg-cell by the characteristic process of 

 segmentation, and the cell-masses so arising differentiate themselves 

 into two layers — the endoderm and ectoderm, or the two primary 

 germ-lamellae *. Resting on the fundamental homology of these 

 two layers in all the six higher types of the animal kingdom, Haeckel 



* The cuinparisou of the eudodcrm and ectoderm of the Coeleuterata to the 

 two primary germ-lamellse of the Vertebrata was first made by Huxley. 



