50 A FURTHER KNOWLEDGE OF THE CYSTIC CESTODES, 



coramon in certain parts of New South Wales. The cysts when 

 fresh appeared as opaque oval bodies, the largest of which niea- 

 sui'ed 5 mm. in length and 4 mm. in breadth. Each cyst encloses 

 a single Cysticercus, which tills up almost completely the cavity 

 of the cyst, in this respect contrasting strongly with the Lialis 

 cyst, in which, as will be seen in the following, the cyst cavity 

 is very large and the one or more Cysticerci lie perfectly free 

 within it. 



Structure of the cyst wall. — In section (Fig. 1) the cyst wall 

 is found to be of considerable thickness and to differ in stiiic- 

 ture in its inner and outer regions. The inner part, which 

 immediately encloses the Cysticercus, is composed of a homo- 

 geneous layer (Fig. 1, h. I.) very similar in appearance and mode 

 of staining to the cuticle of the Cysticercus. External to this 

 homogeneous layer and occupying the central part of the cyst 

 wall is a region (I. l.) having an irregularly laminated appearance 

 and containing large spaces, probably lymph spaces. External to 

 this and forming the outer layer of the wall is a feebly staining 

 region of a fibrous nature (/. I.) and with iiumerous spaces. In 

 the fresh cyst the outermost layers of the wall can be easily 

 removed with needles, leaving the inner layer as a clear mem- 

 brane closely surrounding the Cysticercus. 



Seeing that the cyst wall is marked out by its optical characters 

 and its histological structure into these two regions, I am inclined 

 to attribute to each a distinct origin. The outer region, consisting 

 of the laminated zone and the fibrous zone, is, I believe, derived 

 from the peritoneum of the host by pathological change, while the 

 inner homogeneous layer probably represents a diiect derivative 

 of the six-hooked embryo, and corresponds to the lining of the cyst 

 in Lialis, to the so-called cyst of Monocerci — in a word, to what 

 Villot* terms the blastogen or blastogenic vesicle. 



Structure of the Cysticercus. — When the Cysticerci are liberated 

 from the cyst, they are found to be somewhat pear-shaped bodies 



* "Menioire sur les cystiques des T^nias." Ann. des Sci. Nat. Zool. 

 6nie s6rie, Tome xv. (1883). 



