The Histogenesis of Cysticercus pisiformis. 193 



of the outer layer. Thus it is highlj- probable that a continuity- 

 exists between the fibres of the parenchyma and those of the outer 

 or "hair" layer of the cuticula. This stage is represented in Fig. 17, 

 where the two layers and the continuity of their fibies are clearlj" 

 shown between the developing hooks. The continuity of the pai^en- 

 chyma fibres with the cuticular groundwork is also shown. A 

 further discnssion of the "hair" layer will be taken up in a study 

 of the hooks. The further development of the cuticula consists in 

 the increase in thickness of the inner layer and decrease, or in some 

 places complete loss, of the outer layer. The former assumes a more 

 homogeneous appearance due, not to the loss of its fibrillar nature, 

 since this may be demonstrated in favorable places in the head and 

 neck region of adult larvae, but rather to the more intimate binding 

 together of the fibrillae by the inlaid cement substance. The 

 cuticula in the head and neck region of the larva is markedly 

 thicker than in the same regions of the adult, due probably to the 

 contraction of these parts in the former. 



From the preceding study the following conclusions may be 

 drawn: The cuticula of Cysticercus pisiformis is devel- 

 oped from a groundwork of simple parenchyma fibril- 

 lae by a deposition among them of a cement substance. 

 There are no specialized fibrillae or cellular processes 

 concerned in its development. The fact that in its develop- 

 ment the processes of the sub-cuticular cells take part, does not in 

 any way detract from the above statement, since primitively the 

 sub-cuticular cells themselves are undifferentiated par- 

 enchyma cells. 



A detailed discussion of the various theories relative to the origin 

 and homology of the cuticula will not be undertaken in this paper. 

 Hitherto, the actual development of the cuticula seems to have es- 

 caped observation. The views regarding its structure are as varied 

 as those regarding its origin. Two to four layers have been as- 

 signed to it by various authors, some of whom have included the 

 basement membrane in the cuticula. By most authors it has been 

 described as perforated by numerous fine pores, the pore canals, 

 which have been supposed to contain processes from the sub-cuti- 

 cular cells. By others, the existence of these pores has been denied. 

 Some authors have seen the sub-cuticlar cell processes terminating 

 in the basal portion of the cuticula, which they have described as 

 a differentiated laver for tliis reason. A number of interesting 



