MORPHOLOGY OF EYE MUSCLE NERVES 65 



vanced stages in the histogenesis of the oculomotor and they 

 were consequently led astray in their inductions. 



Filatoff's ('07) assertion that the oculomotor is differentiated 

 from the myotome toward the brain in Emys has not been con- 

 firmed by the more recent investigation of reptile embryos by 

 Johnson ('13). According to a second view — upheld by His 

 ('88), Chiarugi ('97), Neal ('98), Held ('09), Gast ('09), Carpenter 

 ('06), Belogolowy ('10) — the first protoplasmic connection between 

 midbrain and somite 1 is effected precisely as has been found 

 for spinal somatic motor nerves, by the continuous extension of 

 the processes of medullary neuroblasts. Sooner or later, accord- 

 ing to Dohrn ('88, '91), C. L. Herrick ('93), Goronowitsch ('93), 

 Chiarugi ('97), Carpenter ('06) and Gast ('09), this protoplasmic 

 movement is accompanied by a migration of medullary cells 

 from the base of the midbrain. Neither Neal nor Held were 

 able to find convincing evidence of nuclear migration into the 

 oculomotor roots, but they did not deny the fact of migration. 



The evidence presented in sections of Squalus embryos and 

 represented in the figures of plate 6 of this paper, strongly con- 

 firms the opinion of those who have maintained the genetic rela- 

 tions of medullary neuroblasts in the ventral wall of the mid- 

 brain to the first protoplasmic connections between midbrain 

 and the first somite. The anlage of the oculomotor makes its 

 first appearance in Squalus embryos of 9 to 9.5 mm. (fig. 58), 

 as a short, deeply-staining fiber, formed by the union of the 

 processes of medullary neuroblasts. The evidence that this is 

 the anlage of the oculomotor consists in the fact that the point 

 of attachment to the brain, the direction of the long axis of the 

 process, and the histological appearance correspond with those 

 of the oculomotor in a 10 mm. embryo when the connection of 

 the nerve with the myotome is already established. The nerve 

 anlage already consists of two roots, each containing a deeply 

 staining fiber surrounded by an envelope of granular protoplasm. 

 At the point of union of the two roots a mesenchymatous cell 

 appears in close proximity to the anlage. Evidence of the migra- 

 tion of medullary cells is wanting at this stage. The deeply 

 staining processes of neuroblasts at the base of the midbrain 



JOURNAL or MORPHOLOGY, VOL. 25, NO. 1 



