Johnston, The Brain of Petromyzon. 43 



or among the glomeruli themselves, but which forthe most part 

 show no basis for grouping in classes. The most striking fact 

 is that there are no well developed mitral cells to be found. 

 There are, indeed, a few cells situated in the glomerular layer 

 which are somewhat larger than the majority of the cells of the 

 lobe, measuring 12-15 X 17-22//. I have found only a very 

 few such cells in all my preparations and never more than two 

 in any one section although there are scores of other cells fully 

 Impregnated in the same sections. I have inserted into Figs. 

 19 and 20 drawings of four of the most characteristic cells of 

 this type, and* from the drawings it will be seen that they differ 

 from the other cells of the lobe chiefly in the fact that their 

 dendrites are directed outward to a restricted region of the 

 glomerular layer, where they break up into a relatively large 

 and dense end-bush. The branches of this end-bush are inter- 

 woven with the fibers of one of the large olfactory bundles. 

 These are the characteristics of mitral cells in other vertebrates 

 and the cells in question are to be considered as mitral cells, al- 

 though they are very slightly differentiated from the other cells 

 of the lobe. 



The majority of the cells in the lobe are stellate or fusiform, 

 measuring 10-12 X 12-16//, are situated in all parts of the lobe 

 except the glomerular layer, and have two or more dendrites 

 which diverge and break up in widely separated parts of the 

 glomerular layer. Large numbers of these cells are impregnated 

 in all my preparations, and in almost every minute portion of 

 the glomerular layer their dendritic branches are intricately inter- 

 laced, very numerous small branches entering and ending 

 in the glomeruli. These cells are therefore the chief elements 

 for receiving the olfactory stimuli. The neurites of the larger 

 number of these cells pass backward through the lobe into the 

 olfactory area, where they end in relation with the cells above 

 described. The greater part of the course of many of thesfi 

 fibers can be traced in single sections in favorable planes. In the 

 cephalic wall of the lobes many fibers turn towards the middle 

 line and cross to the opposite side in the olfactory commissure. 

 This commissure lies just beneath the extreme cephalic end of 



