204 PROT'. HUXLEY OF THE 



as Opalina ; and I must express my entire agreement with Von 

 Siebold and witli Haeckel in their conclusion, that the protoplasm 

 of these animals is not differentiated into cells. 



At most there is an excessively minute, and sometimes regular, 

 granular structure, which is found in the endoplast, as well as 

 elsewhere, and appears to me to be altogether similar to that 

 of the protoplasm between the nuclei of Opalina. But although 

 the bodies of the Infusoria contain no cells, they may be differ- 

 entiated into very definite tissues. In the genera mentioned, 

 the so-called "cuticula" is, I believe, simply the transparent outer- 

 most layer of the protoplasm, and the cilia are directly continuous 

 with it. Beneath this is a well-marked cortical layer, in which 

 the " trichocysts " of Paramecium are situated, and which, in Spi- 

 rostomum, Balantidium, and Nyctotherus, presents the distinct 

 muscular fibres described by Stein and others. The inner substance 

 is, in some {Balantidium, e. g.), semifluid, and undergoes an obvi- 

 ous rotation ; but in Nyctotherus, not only is there no movement 

 of this substance, but the long curved oesophagus is succeeded 

 by an ill-defined region, which lies between it and the anus, is 

 permanently filled with ingested matter, and is, in one sense, 

 an alimentary tract. Even in Paramecium, the complex water- 

 vessels, which lie, for the most part, not in the cortical layer, but 

 beneath it, show, by the permanence of their disposition, that a 

 great part of the inner substance is fixed. The constancy of posi- 

 tion of the endoplast *, which also lies beneath, and not in, the 

 cortical layer, is evidence to the same effect. 



In comparing the Ciliated Infusoria with nucleated cells, the 

 existence of the so-called " nucleolus," which assuredly can have 

 nothing to do with the histological element so named, and which 

 I propose to term the endoplastula, is an important fact, often left 

 out of sight. 



I have no observation to offer upon the vexed question of the 

 nature of the endoplastula, as none of the numerous individuals 

 of the different species named, which I have examined, showed the 

 changes described by so many observers. That the endoplast 

 itself is a reproductive organ is clear ; but the development of 

 embryos by its fission is an argument rather against, than in favour 

 of, identifying it with the nucleus of a cell. No cell is known to 

 multiply by fission of its nucleus alone. 



* The membranous investment of the endoplast, so often described and figured, 

 certainly has no existence in the unaltered state of the Infusoria I have men- 

 tioned. 



