95 

 in proportion to the -ictuHl magnit.iie of the cell, are rela- 

 tively lar^e ia size, 'f^hen the nucleus lies to«arl one enJ of 

 the cell, the lari^est ti^roids are found in the free extreuity. 

 The prevailing form of tigroid mass is triangular, and t.ie 

 several masses have a rather open arrangement. 



F'rom the form of the neurone just describe I, we should 

 infer that it is adapted to the part of a conducting mediuL 

 bettveen the superficial and the deeper levels of the tectum 

 lesencephali. The internal structure also indicates that it 

 is characterized by no slight amount of metabolic activity. 

 Evidently, then, there is here a nervous element of consider- 

 able importance in the economy of the midbrain. 



VanGehuchten ( '94) nas described the deeper neurones from 

 the optic lobe of the trout (his conohe gra^rj.leuse) as being 

 flithout dendrites, and as sending their axones peripherally 

 into the superficial levels. The character of this type of 

 neurone in Mustelus certainly corresponds the more nearly with 

 the conditions fDund in higher vertebrates, although much sim- 

 pler of course, so that, in this respect at least, the sela- 

 chian is seen to be in the direct phylogenetic line, while the 

 teleost is divergent. 



In the inner zone of the deeper layer of the tectum, at the 

 level occupied by the numerous collaterals from the proper 

 axones, there are to be found at intervals neurones of another 

 character (Pig. 21). Such neurones are irregularly stellate as 

 to general forn, the dendrites anl the axone radiating widely 



