232 Robert Thompson Young, 



a differentiation between nucleus and cytoplasm impossible, while in 

 others a distinct nucleus is clearlj" differentiated from the cytoplasm? 

 Why is it, furthermore, that in some preparations of Taenia sei-rata 

 a distinction, so far as color reaction is concerned, is impossible 

 between the subcuticular cells and those of the subjacent par- 

 enchyma? I shall revert to this question of the staining reaction 

 later. 



In answer to the second objection to the epithelium theory, 

 namely, that the deep position of the sub-cuticular cells imbedded 

 in the meshes of the parenchyma and their consequent separation 

 from the cuticula by muscle layers and basement membrane is not 

 in accord with the true conception of an epithelium, it has been 

 pointed out by Blochmaknt (1896), Hein (1904), Zernecke (1895) and 

 others that in many other invertebrates, notably Hirudo, Arion, the 

 Turbellaria, Oligochaeta and Echinodermata, the epithelial cells are 

 often imbedded in connective tissue elements, blood vessels, etc. It 

 is to be noted, however, that according: to Blochmann's definition 

 cited above an epithelium "entweder selbst die äussere Oberfläche . . . 

 begrenzt, oder auf ihrer Oberfläche eine Membran erzeugt"'. 



Regarding the presence of muscles between cuticula and sub- 

 cuticula, Zernecke (1895: 149) says: "Offenbar ist hier das Ein- 

 dringen der Fasern ein secundärer Vorgang", and a similar view has 

 been expressed by Braun (1894 — 00). This statement is directly 

 controverted by a study of the histogenesis which shows both the 

 sub-cuticular cells and muscles developed in situ. 

 There is never a secondary in-pushing of the muscles 

 between the cells. 



Further, the experiments of Jander (1897) on Dendrocoelmn have 

 been urged as showing a probable inward migration of the sub-cuticular 

 cells from their original epithelial position on the surface, in the 

 Cestodes. I can only say in answer to this inference that what is 

 true in Dendrocoelum is not necessarily true in Cysticercus, and I have 

 positive evidence from my own observations to show that such a mi- 

 gration does not occur in this form. 



My observations do not admit of a full discussion of the third 

 objection to the epithelial theory. In embryo as well as adult, the 

 Cestodes present such an aberrant type of development that it is 

 impossible to homologize their embryonic structures with the germ 

 layers of other animals. Suffice it to say that if an ectoderm is 

 present at all in the Cestodes, it is probably lost in embryonic life. 



