the Ciliate Inf usoria. By G. J. Allman. 181 
which, indeed, he regards as the only true Infusoria — while he 
considers the unicellular ity of the Flagellata as too obvious to 
require an elaborate defence. The value of this paper will be 
obvious from the analysis of it which I now propose to give. 
In stating the argument derived from development, Haeckel 
does not accept as established the alleged sexual reproduction of the 
Infusoria, and he believes it safest to regard as non-sexual “ spores ” 
the bodies ( Keimhugeln ) which result from the breaking up of the 
nucleus, and which Balbiani regarded as eggs. 
These bodies consist of a little mass of protoplasm usually 
destitute of membrane, and including a nucleus within which one 
or more refringent granules admitting of comparison with a true 
nucleolus may sometimes be witnessed — characters which are all 
those of a simple genuine cell. From this spore the embryo is 
developed by direct growth and differentiation of parts ; but however 
great may be the differentiation, there is never anything like the 
formation of a tissue. 
The development of the Infusoria is thus entirely in favour of 
the unicellular theory. This theory, however, is just as strongly 
supported by the study of their mature condition; and here 
Haeckel gives an admirable exposition of the structure of the true 
or Ciliate Infusoria. 
The parts which are common to all Ciliata and which first 
differentiate themselves in the ontogenesis or development of the 
spore, are the cortical layer, the medullary parenchyma, and the 
nucleus, which is situated on the boundary between the two. The 
differentiation of the protoplasm of the naked spore into a clearer 
and firmer cortical substance, and a more turbid, granular, and 
softer medullary substance, corresponds entirely with what we see 
in the parenchyma cells of higher animals. These two products 
of differentiation are designated by Haeckel “ exoplasm ” and 
“ endoplasm.” 
The exoplasm is originally a perfectly homogeneous and 
structureless, colourless hyaline layer distinguishable from the 
turbid granular soft protoplasm of the internal body mass, by con- 
taining in its composition less water, by absence *of included 
granules, and by its high independent contractility. All the mobile 
appendages of the body, the cilia, bristles, spines, hairs, hooks, &c., 
are nothing but structureless extensions of this exoplasm and partici- 
pate in its contractility. In this respect they entirely correspond 
to the cilia and flagellae of the cells which form the ciliated 
epithelium of multicellular animals. 
In many Ciliata we find this cortical layer or exoplasm itself 
subsequently differentiated into distinct strata. In the most highly 
differentiated Ciliata four layers may be distinguished as the result 
of this secondary differentiation of the exoplasm. These are : 
o 2 
