The Histogenesis of Cysticercus pisiformis. 233 



Certainly it is impossible, in Cysticercus pisiformis at least, 

 to homologize the sub-cuticula with an epithelium of ectodermal 

 origin. An outer epithelium, however, is not necessarily an ectodermal 

 structure, for in regeneration experiments on the lower animals 

 it has been shown that originally ectodermal structures may be 

 regenerated from entoderm. I refer to the experiments of Rievel 

 (1896), V. Bock (1898), Haase (1898) and Kkoebek (1900) on Annelids. 

 The fact remains, however, that an outer epithelium is never 

 developed normally from any other layer than the ectoderm. 



The statement of Pe^tner (1889, p. 416, footnote) that the views 

 of those authors who deny to the Cestodes any ectodermal deriva- 

 tives "werden um so unannehmbarer, wenn man bedenkt, zu welchen 

 Consequenzen dieselben mit Rücksicht auf die Entstehung- des Nerven- 

 systems führen", loses its force in view of the fact that this system 

 originates in the parenchyma. 



To recapitulate : I deny the presence of an epithelium in Taenia 

 serrata for the following reasons: 



1. The sub-cuticular cells are merely parenchyma cells modified 

 in situ, retaining their connection with underlying parenchyma cells 

 and in many cases, even in the adult worms, indistinguishable from 

 the latter in any way save position. 



2. Excepting the processes which they send into the cuticula, 

 they are separated from the latter by parenchyma fibres and muscles 

 and hence do not constitute the outer body wall. 



3. The cuticula does not arise from the sub-cuticula, but is 

 developed from the parenchyma before the differentiation of the 

 latter layer. 



4. Cytogenesis and Nuclear Significance. 



Since the days of Schleiden and Schwann the genetic continuity 

 of cells has been an accepted fact, and the statement of Virchow 

 (1855, p. 23) "omni s cellula e ce Hula" has become a biological 

 axiom.^) My observations on cytogenesis in both Cysticercus 

 pisiformis Rud Taenia serrata point very strongly to an exception 

 to this law in these forms, and suggest most forcibly the origin of 



1) The independent or spontaneous origin of cells has been claimed 

 or conjectured by several authors since tbis time ; notably Sheldon (1887), 

 Henking (1888) and Tichomirofe (1894). Henking has more recently 

 however, modified his previous statements. 



16* 



