188 EoBERT Thompson Young, 



brane bounding' the latter, the existence of which has been claimed 

 by some authors and denied b}^ others. 



It may be well at this point to define the term "cell" as used 

 in this paper. By "cell" is meant a nucleus with the protoplasm 

 immediately surrounding- it; since the tissues are sync3^tial in 

 character, adjoining cells can never be definitely delimited from one 

 another. 



Regarding the development of the bladder cavity, I cannot 

 fully agree with Leuckart (1879—86: 434) who says, ". . . diese 

 Finne . . . erst etwa in der vierten Woche, wenn der Wurm bereits 

 eine Länge von 4 bis 5 mm besitzt, in einen eigentlichen Blasen- 

 wurm sich verwandelt. Bis dahin ist dieselbe völlig parenchymatös, 

 im Innern mit einem grossblasigen Schleimgewebe gefüllt, das kaum 

 irgendwo scharf gegen die Eindenschicht sich absetzt und auch 

 gleich dieser von Muskelfasern in verschiedener Richtung durch- 

 zogen wird." 



Cysticercus pisiformis is a bladder worm from the earliest 

 stages of its development in the liver of the rabbit up to the time 

 of its metamorphosis in the intestinal tract of the dog. A bladder 

 cavity is present from the first; its appearance is not delayed till 

 the fourth week of development; and Leuckart's „grossblasiges 

 Schleimgewebe" is nothing more nor less than this same bladder 

 cavity traversed by a few sparse strands of parenchyma. The extent 

 to which the bladder cavity is traversed by these strands of paren- 

 chyma varies in difterent larvae and in different parts of the same larva ; 

 even in the old bladder-worm a few may be found crossing the 

 cavity. Thus the bladder cavity is not separated by a definite 

 membrane from the parenchyma wall, as is claimed by Schaaf (1905), 

 but is ahvays in communication with the parenchyma spaces of the 

 latter and these on their part communicate with one another. 



To return now to the consideration of Fig. 14: the wall is 

 composed of a close interlacing of parenchyma strands running in 

 diverse directions. At this stage, the presence of a cuticula cannot 

 be demonstrated. It seems most likely that a cuticula has not yet 

 been developed. Studied under a 2850 magnification, the bladder 

 wall shows no continuous membrane surrounding it, but a succession 

 of irregular spaces communicating with the bladder cavity on the 

 one hand and the exterior on the other (Fig. 7). A somewhat older 

 larva enclosed in its cyst is shown in Fig. 1. Here the principal 

 points to notice are the enlargement of the bladder cavity relative 



