Johnston, The Radix Mesecenphalica Trtgemini. 639 



true axones with, central distribution is what we should expect in 

 ^dew of the fact that the peripheral process goes into the sensory 

 nerve and is therefore the dendrite, and in view of the disposition 

 of the processes of the giant or dorsal cells of the spinal cord which 

 is well understood. I am satisfied that such central axones entering 

 the substance of the tectum are not uncommon in fishes and amphi- 

 bians. If the bipolar (and multipolar) cells in mammals are inter- 

 preted in the most simple and direct manner, they must be placed in 

 the same category. But the majority of cells in mammals seem to 

 have only a single process which sends collaterals into the motor 

 nucleus of the trigeminus. Are these collaterals to be regarded as 

 the central axone and the point of their origin as the T-division of 

 the single process of the ganglion cell ? I should hold this hypothesis 

 in doubt until we have a very thorough knowledge of these neurones. 

 It involves the supposition that these neurones begin as bipolar cells 

 (already kno^vn), change into unipolar cells as do the spinal ganglion 

 ceUs, and that the single process grows to an enormous length in 

 case of those neurones whose cell bodies lie in the tectum. On the 

 other hand, Van Gehuchten figures the collaterals from the large 

 processes at the level of the motor nucleus of the trigeminus in the 

 trout, while from his figures and from my sections of Acipenser and 

 selachians I believe that the cell bodies in the tectum bear true axones. 

 The only other case in the vertebrate nervous system which comes to 

 my mind in which axones or axonic collaterals are given off from the 

 afferent or dendritic process of a neurone in addition to an axone 

 arising from the cell body, is the case of the giant cells in the spinal 

 cord of fishes (l. c, p. 376). These neurones possess ascending 

 axones arising from the cell body and descending axones arising from 

 the dendrite. (For other conditions occurring see the paper referred 

 to.) The motor collaterals in the case of the trout (Van Gehuchten) 

 and mouse (Cajal) would seem to correspond closely to the descend- 

 ing axone arising from the dendrite of the giant cells in the cord of 

 teleosts. This suggests the hypothesis that what may be called a 

 descending or accessory axone has grown in importance in higher 

 forms, while the true axone ending in the tectum has been reduced and 

 possibly is absent from most of the cells in mammals. On this 



