No. 3.] STUDIES ON THE HETERONEMERTINI. 389 



nerve chord: CI. II, the cell clusters; P, their respective open- 

 ings in the inner neurilemma ; x-x, the imaginary median plane 

 of symmetry. 



C. Cell III 



a. Lineus. — These cells (Figs. 3-16) are of an elongated 

 pyriform shape, largest and rounded proximally, seldom nearly 

 spherical. It may be. noted that while the cell bodies vary 

 considerably in size, their nuclei remain of nearly uniform 

 dimensions. 



The nucleus (A^) seldom lies in the proximal portion of the 

 cell, but usually near the center, or even in the constricted, 

 distal portion; this would show that it, as well as the nucleus 

 of I {v. supra), stands in as close a physiological connection 

 with the substance of the nerve tubule, as with the cytoplasm 

 of the cell body itself; this fact might well be borne in mind 

 by those who suppose the vitality of the cell process to be 

 more or less distinct from that of the cell. The nucleus is 

 usually spherical, and larger than those of any other somatic 

 tissues, except a few of those of the outer neuroglia cells 

 (Montgomery, '97). Its chromatin is more or less regularly 

 distributed in granular form, but occasionally produces a thick 

 peripheral layer, from which a few chromatic fibres pass 

 towards the nucleolus. Such reticulation of the chromatin is 

 especially discernible on preparations fixed with a fluid contain- 

 ing OSO4; while the action of corrosive sublimate obliterates 

 to a great extent whatever reticulation may exist, and imparts 

 a granular appearance to the chromatin. One nucleolus (;/) is 

 always present ; occasionally there are two, and then of unequal 

 size; the single nucleolus is most frequently centrally situated, 

 and then is enveloped by a sheath of chromatin. 



The cytoplasm is, as a rule, coarsely vacuolar (vesicular), 

 especially so towards the distal pole. A thin peripheral layer 

 of finely granular cytoplasm is always present (Figs. 3-16, Ah); 

 this layer might compensate physiologically for the absence 

 of a cell membrane, as has been suggested by Leydig ('64). 

 Burger ('90b), who has also noticed this layer, described it as 

 more coarsely granular than the rest of the cytoplasm. This 



