226 Robert Thompson Young, 



ray views are quite the reverse of those held by Blochmann and 

 his followers, I feel that I ought at this point to pause for a brief 

 consideration of this much disputed question. The importance of 

 considering this question from the ontogenetic viewpoint has been 

 indicated by both Bkaun (1894—1900), and Blochmann (1896, p. 11), 

 from whom I quote the following: "Man wird vor Allem auch die 

 Entwickelung der Plathelminthen von histogenetischen Gesichts- 

 punkten aus untersuchen müssen." 



A detailed account of the opinion of all the authors who have 

 considered this subject will not be given here. For such an account 

 the reader is referred to Braux (1894 — 1900), Blochmakn (1896) 

 and Hein (1904). A brief outline of the principal views which have 

 been expressed will, however, be given, followed by a somewhat 

 more detailed account of the writings which have appeared since 

 the publication of "Die Epithelfrage" by Blochmann (1896), inclusive 

 of this article. 



The definition of an epithelium as given by Blochmanx (1896, 

 p. 3) will be taken as the basis of the following discussion: "Ich 

 verstehe also unter äusserem Epithel eine Zellenschicht, die entweder 

 selbst die äussere Oberfläche des Thierkörpers begrenzt, oder auf 

 ihrer Oberfläche eine vom Zellplasma chemisch mehr oder weniger 

 différente Membran erzeugt, die dann ihrerseits den äusseren Über- 

 zug des Körpers bildet. Wie diese Membran gebildet wird, durch 

 eine Art von Secretion, oder durch chemische Umwandlung der 

 peripheren Plasmapartien ist für unsere Frage einerlei; ebenso ob 

 diese Membran besondere Structurverhältnisse hat, oder nicht." 



Briefly stated, the following are the views which have been 

 held concerning the homology of the sub-cuticula, basement membrane 

 and cuticula: 



1. The cuticula represents the basement membrane of an 

 epithelium which has been lost ontogenetically. 



2. The cuticula is a metamorphosed epithelium. 



3. The cuticula is deposited by the sub-cuticular cells, which 

 are modified parenchyma cells. 



4. The cuticula is deposited by the sub-cuticular cells, which 

 form a true epithelium. 



The first two of these theories may, so far as my own investi- 

 gations are concerned, be passed over with the mere mention. In 

 the development of Cysticercus pisiformis, there are at no 

 time any phenomena affording ground for their support. It is true 



