222 JOHNSTON. [VOL. I. 



acterized, in preparations stained with haematoxylin, by the 

 compact masses of branching dendrites and nerve fibers known 

 as olfactory glomeruli. This zone constitutes about one-third 

 of the thickness of the wall of the lobe. The granular zone is 

 made up of numerous nerve cells and fibers. There is no dis- 

 tinct white zone. The axis cylinders of the cells of the lobe, 

 as will be shown later, take the shortest course toward the 

 fore-brain. In consequence, the granular zone is a mixed white 

 and gray zone. It is readily distinguished from the glomerular 

 zone, in ordinary preparations, by the absence of the glomeruli; 

 but the line of division between the two zones is not sharply 

 defined. The central cavity of the lobe is large and communi- 

 cates widely with the cavity of the fore-brain (Fig. i, B}. 



a. ZONE OF OLFACTORY FIBERS.- -The fibers coming from 

 the sense cells of the olfactory epithelium are gathered in bun- 

 dles from 80 to 400 p in thickness (Fig. i, o.f.z.). The indi- 

 vidual fibers differ considerably in thickness. They are very 

 direct in their course and present definite varicosities at irreg- 

 ular intervals throughout their entire length. There is no 

 evidence that the appearance of varicosities is due to imperfect 

 impregnation, as suggested by Van Gehuchten and Martin ('9i). 

 (I learn from Miss Langdon that in her forthcoming paper on 

 Nereis virens she will present evidence to show that such vari- 

 cosities are artifacts.) The bundles break up into smaller 

 bundles as they enter the gray matter. 



b. ZONE OF OLFACTORY GLOMERULI. - - The olfactory fibers 

 approach their termination in the glomerular zone either in 

 small bundles or singly. Occasionally the fibers divide shortly 

 after entering this zone and end in two or more glomeruli, but 

 the greater number divide only in the glome~ruli in which they 

 terminate. I shall consider the glomeruli after the various 

 elements which may contribute to their formation have been 

 described. 



I have found in the glomerular zone three distinct forms of 

 cells : mitral cells, stellate cells, and cells with short axis 

 cylinders. 



(i) There are present mitral cells of two varieties. The first 

 (Fig. I, g} are large, measuring from 16 to 48 /u in their short 



