Johnston, The Radix Mesencephalica Trtgemini. 607 



search may reveal such branching in some of the cells, but there is 

 reason to think that the collateral branching described by Lugaro and 

 Cajal is the chief form of branching in mammals. 



In addition to the protoplasmic processes or dendrites these cells 

 have in some forms at least a line process which must be regarded as 

 the true axone. The description of bipolar cells with ascending 

 axones by Merkel and Krause must be given some weight, although 

 Cajal subsequently saw only the descending thick process. It is 

 possible that not all the cells are alike, that some of them are unipolar 

 (Cajal) and others multipolar (Kolliker, Held) and still others 

 bipolar with differentiated dendrite and neurite (Merkel and 

 Krause). It is certainly true that in the toad both multipolar and 

 bipolar cells exist and that in both forms a typical slender axone 

 can be disting-uished from the thick process (and from the small 

 dendrites of the multipolar cells). A considerable proportion of the 

 cells in the toad brain prepared by the Cajal ammonia-silver method 

 show clearly typical axones (Fig. 7). I have drawn only cases in 

 which the thick process and the axone can be seen in the same section, 

 but there are many cases in which the processes of the same cell are to 

 be found in adjacent sections. The characteristics which show the 

 process in question to be an axone are its typical cone of origin, its 

 slender uniform caliber and its deep staining. The neurofibrillse in a 

 few cases are seen in the cone of origin converging into the fiber. The 

 large process which has heretofore been called the axone is broad, 

 pale and tapering so that it has the appearance of a dendrite until 

 it has gone some distance from the cell and taken on its myelin 

 sheath. As seen in the figures, the axone may arise from the cell 

 body of a stellate cell bearing one or two small dendrites, or from 

 one pole of a spindle-shaped cell whose other pole gives rise to the 

 thick dendrite, or from the dendrite itself a short distance from the 

 cell (Fig. 7). In every case in which a true axone was found the 

 thick process entered the descending bundle and the axone pene- 

 trated the substance of the tectum itself. 



In the rabbit (Fig. 5 D) an occasional bipolar cell is seen whose 

 slender axone takes an ascending direction. 



In ordinary stains the cell body is large, pale, uniformly granular. 



