86 H. V. NEAL 



sented in figure 53, which shows nuclei half in and half out of 

 the brain wall at the point of emergence of the trochlearis root. 

 Dohrn (p. 293) also called attention to the fact that the nuclei are 

 not peripheral in position but lie between the fibers of the nerve 

 root. Furthermore, medullary nuclei lie nearer the periphery of 

 the brain wall in the region where the fibers of the trochlear enter 

 the brain than they do elsewhere in the vicinity of the root, and, 

 as development goes on, an increasing number of nuclei appear 

 among the fibers of the nerve root. More convincing evidence 

 of migration would be difficult to find in sectioned material. 



Belogolowy ('10a, p. 375) suggests that the absence of ac- 

 companying cells in the fiber bundle of the trochlear anlage in its 

 intracerebral and in its proximal extent proves that the accom- 

 panying cells are not medullary in origin. However, had Belo- 

 golowy studied somewhat more advanced stages with this problem 

 in mind, it is posssible that he would have found evidence of med- 

 ullary migration. The writer ('98), on grounds similar to those 

 advanced by Belogolowy, concluded that medullary elements 

 were absent from the trochlear anlage of Squalus, but renewed 

 investigation of more advanced stages proves this inference to be 

 erroneous. 



On the other hand, evidence that mesenchymatous cells attach 

 themselves to the nerve anlage is diflficult to obtain and positive 

 proof is altogether wanting. Belogolowy ('10a) infers the par- 

 ticipation of mesenchymatous cells in the formation of the neuri- 

 lemma of the trochlear. The most convincing evidence that 

 may be secured is possibly that mesenchymatous cells lie in va- 

 rious degrees of proximity to the nerve anlage (fig. 51). In other 

 words the evidence of the participation of the mesenchyma in 

 forming the cellular elements of the nerve anlage are quite as 

 obscure as in the case of spinal somatic motor nerves. 



Miss Piatt ('91) was the first to observe evidence of migration 

 of cells from the anlage of the superficial branch of the trigem- 

 inal into or towards the trochlear anlage. The evidence is 

 similar to that seen in the relations of the oculomotor to the pro- 

 fundus ganglion. Miss Piatt, after describing (p. 95) the first 

 appearance of the trochlearis as a "small fibrous nerve growing 



