Structure and Development of the Nepbridia. 369 



excretoiy functiou they are mucli vaciiolated and contain uuraerous 

 deeply staiuiug- g-rauules. Tlie uucleus is sìtiiated in the middle 

 regiou of the celi, usually soniewliat nearer the base than the outer 

 border. It is larg-e, geuerally roimded in shape biit frequenti} of 

 irregulär outline, and contains a chromatin network of the appearance 

 represented in Piate 22, Fig-. 5, together with a conspicuous deeply 

 staiuing nucleolus which in nearly every instance is seen to contain 

 a well-defined vacuole. The vacuolation of the nucleolus is character- 

 istic of the excretpry cells, and also, as will be poiuted cut later, 

 of the large nuclei of the g-rowing zone, and of the glandulär cells 

 of the veutral wall of the stomach. The peculiarity seems to be 

 associated with unusual metabolic activity. It is noteworthv that in 

 the excretory celis the position of the nucleus always closely corres- 

 punds with that of the zone of deepl}^ staining excretory grauules. 



— The ditferent regions of the celi show very Constant and character- 

 istic peculiarities, as may be seen from Fig. 5 which represents a 

 portion of the excretory epitheliiim seen in cross section. At its 

 base each celi shows typically a denser structure than at the more 

 peripheral portions, and in fìxed and stained preparatious, seems 

 composed chiefly of a feltwork of prevailingly lougitudiual tibrils 



— an appearance which seems characteristic of ciliated cells. 

 Typically the basai part of the celi is almost free of granules. 

 Towards the middle regiou where the nucleus is situated, the 

 protoplasm becomes more vacuolated and is in most instances largely 

 fìUed with miuute round granules which stain an intense black 

 with iron-haematoxylin. Besides these deeply staining granules, 

 tbere are others (which in some cells may preponderate : see Fig. 5) 

 which stain less deeply and seem to bave a greater affinity for the 

 acid stains such as erythrosin. The restriction of the deeply staiuing 

 granules to the middle regiou of the celi is ver}' Constant, though not 

 absolutely so, and gives rise in cross section to the appearance 

 represented in the text-figure (p. 362). The number of granules, 

 however, shows great variability, some cells being almost free of 

 them while others are so deeply laden as to preseut an almost uui- 

 formly black appearance. The extreme outer portion of the celi is 

 usually, but not always, free of black granules, and is composed 

 of a tìnely granular, much vacuolated protoplasm which frequently 

 contain numerous lightly staining rounded bodies, apparently of the 

 same nature as the erythrophilous granules just descrìbed as present 

 in the middle regiou of the celi. These bodies range in size from 



