the Ciliate Infusoria. By G. J. Allman. 187 
which the body of the ciHate 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 
difference, as Haeckel with much force points out, between the 
differentiation process of these parenchyma cells and that of the 
Infusorium body consists in the fact that in the parenchyma cells 
the differentiation is a one-sided one, conditioned by the division of 
labour in the organism of which they form the constituents, while 
in the Infusorium it is a many-sided one related to all the different 
directions in which cell-life manifests itself, and resting on a physio- 
logical 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 formation 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 determines 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 in- 
fluence on the rest of the organization, and such "multicellular ciliata" 
are to be compared with the colony-building forms of the Acinetae, 
Gregarinae, Flagellatse, 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 
developed 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 the ectoderm, or the two 
primary germ lamellae.* Eesting on the fundamental homology of 
these two layers in all the six higher types of the animal kingdom, 
Haeckel had already t directed attention to the fact that all these 
types pass in their development through one and the same remark- 
able form, to which he gives the name of Gastrula, and which he 
regards as the most important and significant embryonal form of 
the whole animal kingdom. This gastrula consists of a multi- 
cellular, usually oviform uniaxial, body enclosing a simple cavity — 
* The comparison of the endoderm and ectoderm of the Coelenterata to the 
two primary germ lamellse of the Vertebrata was first made by Huxley, 
t ' Die Kalkschwamme,' 1872. 
