80 H. V. NEAL 



in their application to the growth of the trochlearis. The chief 

 axes of the neuroblasts which form the trochlearis fibers do not 

 point toward the end-organ, neither do they follow the shortest 

 possible course, but practically the most indirect route possible. 



In Squalus embryos, previous to the first appearance of the 

 trochlear anlage, the region of its future path is filled with a 

 loose mesenchyma derived at least in part from the neural crest. 

 It is a most important fact, demonstrated by the observations of 

 Kastschenko ('88), Neal ('98) and Dohrn ('07), that, from the 

 time of its first appearance to the time it attains connections 

 with the superior oblique muscle, the trochlear anlage in some 

 selachians shows no close associations with mesenchymatous cells, 

 but extends as a loose bundle of fibers through vacuolar spaces 

 (fig. 49). The trochlearis development, says Dohrn ('07, p. 413) 

 "affords a proof that an elongated motor nerve can grow through 

 a considerable extent of tissue without becoming attached to a 

 single cell of the surrounding mesoderm." Evidence of a granular 

 protoplasmic envelope surrounding the fibers, however, is not 

 lacking, but there is no evidence that surrounding mesenchj^ma- 

 tous cells contribute to the production of this. The descriptions 

 of Froriep ('91) and Miss Piatt ('91) relate to more advanced 

 stages in the histogenesis of the trochlear than those described, 

 so that their assertions of the participation of cells in the formation 

 of the trochlearis anlage have no relevancy in connection with the 

 problem of the formation of the primary connection between tube 

 and myotome. 



b. What cells participate in the formation of these connec- 

 tions? The fact that in the adults of all classes of vertebrates the 

 fibers of the trochlearis may be traced in suitable preparations to 

 a nidulus in the somatic motor column, posterior to the midbrain 

 leads to the supposition that these cells are genetically related 

 to the fibers. The further evidence that during the period of ex- 

 tension of the fibers toward their terminations in the superior 

 oblique muscle, no cells which may be regarded as neuroblasts 

 or 'nerve-cells' in the Apathy sense, may be seen in connection 

 with the peripheral stretch of nerve anlage, points in the same 

 direction. The connections with cell-clusters or fragments of 



