78 H. V. NEAL 



3. Does the histogenesis of the trochlearis resemble that of a 

 somatic motor spinal nerve? 



a. Are protoplasmic connections between myotome 2 and hind- 

 brain primary or secondary? The distance between the point of 

 emergence of the trochlearis fibers and the myotome which this 

 nerve innervates is, in Squalus embryos, the greatest traversed 

 by any motor nerve. This fact lends especial interest to the de- 

 velopment of this nerve in relation to the problem of the existence 

 of primary paths or connections between nerve center and the 

 end-organ. This nerve would seem to afford a critical test of 

 the truth or falsity of the Hensen-Held hypothesis of nerve his- 

 togenesis. If any nerve in the vertebrate body needs a primary 

 path so that it may not go astray, that nerve is the trochlear. In 

 view of this fact it is somewhat surprising that Paton ('07), who 

 supports this hypothesis, concedes that the trochlearis (and also 

 the oculomotor) acquires its connection with the superior oblique 

 muscle without the participation of plasmodesmatous bridges 

 (Paton '07, p. 556). 



Some years ago ('98, p. 237) I expressed the opinion that 

 ''the possibility of a primary connection between muscle and 

 nerve appears excluded in the case of the superior oblique mus- 

 cle and the trochlear nerve" and Dohrn, later ('07, p. 410), said 

 quite as emphatically that "Jede Moglichkeit eines uranfangli- 

 chkeit Zusammenshangs zwischen Muskelzelle und Nervenzelle 

 ist dabei ausgeschlossen. " Belogolowy ('10a, p. 380) also agrees 

 in affirming the absolute impossibility of a primary connection 

 between nerve and muscle in the case of the trochlearis. While 

 Sewertzoff ('98) and Fiirbringer ('02) are unable to agree with 

 this assertion of mine, they advance no arguments in its rebut- 

 tal. It is a significant fact that none of the supporters of the 

 Hensen hypothesis have been able to demonstrate a primary 

 protoplasmic path connecting the isthmus or the hindbrain with 

 the myotome of Van Wijhe 's second somite. 



On the contrary, and in support of the view that the trochlear 

 nerve develops in accordance with the process theory of neuro- 

 genesis and that the connection of hindbrain and superior oblique 



