628 journal of Comparative Neurology ajid Psychology. 



roots of the hypoglossal. With the finding of these ventral roots 

 between the hypoglossal nerve and the abducens, it seems to me 

 that we can no longer consider this theory, but must return to the 

 conception of the cranial nerves as modifications of the spinal 

 nerves, but essentially similar to them. The presence of embry- 

 onic ventral roots having the same relations to the glossopharyngeal 

 nerve as do the hypoglossal roots to the vagus and accessory com- 

 plex (i. e., passing just behind the trunk to reach the outer side) 

 further strengthens, if it does not prove the older theory. For, 

 if these ventral roots are vestigial, as I think, we have evidence 

 that the glossopharyngeal nerve originally possessed the same 

 roots as a spinal nerve. 



The loss of the ventral roots between the abducens and hypo- 

 glossal nerves in the adult should be explained by the absence of 

 muscles derived from the mesodermic segments in this region; 

 and I am led to believe that this reduction in the number of ventral 

 roots is quite extensive, including some of the anterior roots of 

 the hypoglossal nerve. If we examine figs, i, 2 and 3, the first 

 two of a 10.2 mm. embryo, the last of a 22.0 mm. embryo, we 

 notice that the relative distance between the abducens and hypo- 

 glossal roots is greater in the older embryo. This might be 

 accounted for by the growth of the medullary floor between 

 these two points, but, it seems to me, is really due to the loss of 

 some of the more anterior hypoglossal roots. In figs, i and 2 at x 

 we can see the process going on; and in a reconstruction of a human 

 embryo of 13.6 mm. now being prepared by F. W. Thyng, the 

 same process of degeneration of a hypoglossal root is figured. 



We should conceive, therefore, a row of ventral roots from the 

 medulla, in mammals, originally arising as far forward as the 

 abducens, belonging, so to speak, with the vagus-accessory com- 

 plex, the glossopharyngeal nerve, and possibly with the facial 

 nerve; but soon disappearing because of the failure of the seg- 

 mental musculature to develop in the head region between the 

 hypoglossal muscles and the eye-muscles. Such a continuous 

 row exists in the young of the cyclostome Bdellostoma (Johnston 

 1906, p. 190). These vestigial roots are found in sixteen of 

 the twenty-seven human embryos studied, and occur frequently 

 in the embryos of pig, rabbit, sheep, and dog, though not found in 

 the cat and opossum embryos of the Harvard Embryological 

 Collection. They are found almost constantly in the turtle and 



