MORPHOLOGY OF EYE MUSCLE NERVES 87 



from the constriction between midbrain and hindbrain, " says 

 that ''soon after the appearance of this small nerve, which is 

 the root of the permanent trochlearis, cells are proliferated to 

 meet it from the ganglion cells that lie above the superior oblique 

 muscle." These cells cannot rise from the brain, since "no 

 cells are found in the root of attachment." Thus the permanent 

 trochlearis arises from two sources from the brain and from the 

 ganglion cells." 



Dohrn ('07) also inferred that the first cells connected with 

 .the trochlear anlage in Pristiurus are derived from the ramus 

 superficialis V, while in those forms in which, as in the Torpe- 

 dinidae, the superficial branch is not differentiated, but is rep- 

 resented by fragments of the neural crest, the trochlearis anlage 

 acquires relations to these cell clusters, similar to those in Squalus 

 and Pristiurus, and presents similar evidence 'of cellular migra- 

 tion toward the nerve anlage. 



The evidence of cell migration from the R. superficialis V 

 into the trochlear anlage in Squalus consists of the fact that 

 cells appear primarily, not in the proximal root of the nerve nor 

 in the bundle of fibers, but in the distal portion of the nerve in 

 the region where its fibers cross those of the superficialis. The 

 phenomena presented are practically identical with those described 

 for the oculomotor in its cellular relations with the R. prof. 

 Trig. As development proceeds, cells increase in number in 

 the region between the two nerve anlagen, although the proximal 

 portion of the nerve remains free from cells. Finally, cells ac- 

 cumulate between the trochlear and superficialis anlagen in a 

 mass precisely comparable to the anlage of the ciliary ganglion, 

 forming the cell cluster to which Miss Piatt ('91, p. 100) refers as 

 the ganglion of the trochlearis lying above the superior oblique 

 muscle anlage. 



Further evidence of migration into the trochlear anlage from 

 the superficiahs nerve is found in spindle-shaped cells closely ap- 

 plied to the fibers of the trochlear anlage in stages when cells are 

 wanting in the proximal portion of the nerve and when evidence 

 of additions from the mesenchyma is lacking. Similar spindle- 

 shaped cells show connections, on the one hand with the ramus 



