326 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Small bipolar cells are also present, uniformly distributed. 



Though large multipolar cells are abundant at the ends of the commissures, 

 there are few ganglion cells in the chiasmas. 



In glycerine the nerve fibrillse appear finely granulated and possess strong 

 refractive power. Generally speaking, they are uniform in thickness, about 0.001 

 mm. in cross section. 



Two entirely distinct kinds of ganglion cells are found among these nerve 

 fibrillse, small bipolar cells 0.003 mm. long irregularly distributed between the 

 fibers, and larger, mostly multipolar, cells which are either spindle shaped 

 (bipolar) or very irregular. The size of the latter is difficult to give, for the cell 

 body may be much elongated; according to Hamann, they vary between 0.007 mm. 

 and 0.02 mm. Reichensperger says that, including their processes, they vary from 

 36 n by 10 [J. to 65 \t. by 10 ia. Their vesicular necleus is oval and almost always 

 shows a prominent nuclear body and a fine, more or less diffused, network of chro- 

 matin threads. The cytoplasm stains very deeply and appears granulated, the 

 granulations being traceable to the extreme end? of the processes. 



While the smaller ganglion cells are scattered everywhere in great numbers, 

 the larger, less numerous, chiefly lie on the surface of the fibrillar bundles, being 

 especially abundant about the base of the lateral nerve branches, particularly at 

 the chiasma which is formed at the commencement of each pinnule nerve. In the 

 arms they may pass deeply into the nerve cord and there form a longitudinal 

 band which often makes the cord appear as if divided into two halves by a median 

 septum. 



The processes of the ganglion cells ramify and penetrate the fibrillar bundles. 



Branching of the axial cord in the arms. 



In the middle of each brachial there is a swelling of the axial nerve cord, and 

 from this swelling four stout nerve branches arise with laterally and dorsally a 

 few weaker additional branches between them. 



The individual nerve branches leaving the mid-brachial swelling in part pene- 

 trate far into the interior of the nerve mass of the central cord, and they are bound 

 together at the base by commissures. 



Of the four chief nerve branches the two dorsal penetrate the calcified 

 substance of the brachials and reach the epithelium; the two ventral pass to the 

 interbrachial muscles, the ventral epithelium, and the pinnules. 



Hamann has worked out in Heliotnetra glacialis the ramifications of the two 

 dorsal nerve branches. Shortty after their origin these di^dde dichotomously, 

 and each of the resultant twigs di\'ides in the same way. This process continues 

 until each nerve presents the appearance of a widely branched tree of which the 

 trunks are the original dorsal nerve branches and of which the outermost twigs, 

 composed each of a few fibrillfe, enter the body epithelium. 



The ultimate divisions of these dorsal nerve branches, composed of small 

 bundles of nerve fibrillje, lead to specialized groups of epithelial cells. The cylin- 

 drical or more or less flattened epithelial cells in these groups are more crowded 

 than usual, so that the nuclei appear more numerous. They taper toward their 



