210 • Robert Thompson Young, 



exists as to whether they should be considered as a part of the 

 parenchyma or as forming a special epithelial layer. For a summary 

 of the diiferent views on this subject, the reader is referred to the 

 paper of Bugge (1902, p. 179). Bugge himself in maintaining- the 

 latter view says (p. 181) "Ausläufer dieser Epithelzellen der Wand 

 . . . gegen das Parenchym habe ich nicht beobachtet." Bugge's 

 flg. 3, however, contradicts this assertion. Inasmuch as essentially 

 the same condition, as regards an epithelium or its absence, exists 

 in the excretory ducts as in the sub-cuticula, a further consideration 

 of these views will be postponed until later. 



f) The Nervous System. 



At the outset of the account of the development of this system, 

 it may be well to separate the types of cells which are to be 

 discussed into two classes: namely, the ganglion cells and the 

 peripheral nerve cells. The former are nerve cells appertaining to 

 what may be called, in accordance with the terminology of Cohn 

 (1898), the "central nervous system", regardless of whether they 

 are grouped in ganglia or, as is frequently the case, scattered along 

 the nerve cords; while the latter comprise all the other nerve cells 

 including the "myoblasts" or neuro-muscular cells. 



The precise time at which the development of the nervous 

 system begins is difficult to determine. The first trace of it consists 

 of a somewhat elongated mass of closely packed cells in the posterior 

 part of the scolex rudiment during the early development of the 

 hooks and previous to the appearance of the suckers. Shortly after 

 the appearance of the latter, the rudiments of the two lateral 

 cephalic ganglia^) and the main nerve cords connected with them 

 become apparent. A cephalic ganglion at this stage consists of a 

 mass of closely approximated cells which are rapidly elaborating 

 granular protoplasm in which new nuclei are developing (Fig. 65). 

 The close grouping of these neurogenic cells arises as a result of 

 rapid multiplication and not by any approximation of already existing 

 cells. At least there is no evidence suggesting the latter method, 

 while the former is very evident in many places. This figure shows 



1) In the following discussion, the terminology of NlEMlEC (1885) 

 for the different parts of the nervous system of Taenia serrata will be 

 employed. For a diagram of the nerves in the scolex see his fig. 3, 

 tab. 18. 



