Johnston, Morphology of the Head. 213 



shifts back from neuromere ix to neuromere xi. Such an amount 

 of shifting of the relative position of the auditory vesicle 

 and brain can scarcely have failed to influence the segmental 

 position of the nerve roots. Hill's figures (52) also show a 

 slight shifting of the auditory vesicle during development in 

 teleosts. The relative position of the auditory vesicle and the 

 nerve roots also changes, as indicated in part above. From 

 lying opposite a blank neuromere between Nn. V and VII in 

 the embryo of Petromyzon, in the lower Gnathostomes it 

 comes to lie between the roots of Nn. VII and IX which are 

 crowded apart by the vesicle although they remain connected 

 with successive neuromeres. In reptiles, birds and mammals the 

 vesicle is found between Nn. VII and IX and there is a blank 

 neuromere opposite it, in addition to the blank between Nn. V 

 and VII. It is a noteworthy fact that in mammalian embryos 

 the VII-VIII ganglionic complex lies wholly in front of the 

 vesicle, while in. lower vertebrates it lies between the vesicle and 

 brain. 



If we couple with this shifting of the auditory vesicle a 

 tendency to concentration of the centers for the V, VII and X 

 nerves, I think we shall find sufficient cause for the observed 

 arrangement of the nerve roots-. The visceral sensory fibers 

 form a descending tract {^fasciciilus cornniunis, f. solitaritis in man) 

 and end in relation to cells accompanying the tract and in the 

 nucleus co.mmissuralis at the caudal end of the oblongata. The 

 general cutaneous fibers run caudally in the spinal V tract. The 

 motor roots are formed by fibers which arise from nuclei caudal 

 to the place of exit of the roots and run forward parallel with 

 the somatic motor fasciculus. Under these conditions it is 

 reasonable to suppose that there may have been a shifting back- 

 ward of the nerve roots toward their centers. The efficiency 

 of this influence seems especially clear in the case of the com- 

 munis components, many of which primitively decussated in 

 the dorsal commissure of the oblongata (Sec. 17). As the 

 formation of the choroid plexus crowded this commissure back- 

 ward to the present position of the commissura infima Halleri, 



