70 H- '^'- NEAL 



for his conclusion was that cells of the midbrain migrate periph- 

 erally and then send out processes, which unite in a network, 

 just outside the base of the midbrain, to form the stem of the 

 nerve. Immediately at the beginning of the plasma outflow, 

 cells are seen half in and half ovit of the wall of the tube, and 

 later, before the oculomotorius shows any connection with the 

 mesocephalic ganglion, large deeply-staining nuclei, like those 

 which appear in the process of emergence from the neural tube, 

 make their appearance in the network of the roots of the nerve. 

 In later stages the nuclei, in increasing numbers, appear to 

 emerge from the midbrain into the nerve anlage. The phe- 

 nomena of medullary migration into spinal somatic motor nerves 

 add support to Dohrn's contention. 



Evidence similar to that presented by Dohrn ('91) may be 

 found in sections of Squalus embryos in stages from 10 to 15 

 mm., and confirms the accuracy of his description. The cells 

 closely associated with the oculomotor in the earliest stages of 

 development, appear to be mesenchymatous rather than medul- 

 lary in origin, but it seems not unlikely that these, like similar 

 mesenchymatous cells associated with spinal somatic motor nerves 

 in the very earliest stages of their appearance, are not perma- 

 nently associated with the nerve. Their relations may be purely 

 topographic and transient. But there is no reason to doubt 

 that some of the cells of the oculomotor anlage have, like those 

 of spinal nerves, a medullary origin. 



According to Hoffmann ('85), Ewart ('90), Miss Piatt ('91), 

 Mitrophanow ('93), Sedgwick ('94), Chiarugi ('94, '97), Neal 

 ('98), and Gast ('09) cells migrate into close relations with the 

 oculomotorius anlage from the mesocephalic ganglion. I was 

 not sure that these cells came into permanent relations with 

 the nerve, while Gast inferred the derivation of some of the 

 sheath cells from them. Harrison's demonstration, through ex- 

 periments upon amphibian embryos, of the derivation of neuri- 

 lemma cells from the neural crest might appear to support Gast's 

 inference, although nerve histogenesis in amphibia is not neces- 

 sarily analogous in all features with that in selachians. Gast's 

 evidence of the forward and back — centripetal and centrifugal — 



