MOTOR NUCLEI IN PHYLOGENY 63 



column as in Rana catesbiana, the limits of which are sufficiently 

 well defined to enable it to be indicated diagrammatically in 

 figure 11, C. The intrinsic difTentiation of this cell group in 

 Damonia presents little or no advance over the condition obtain- 

 ing in Rana (5, figs. 1 and 2). 



From their nucleus of origin the XII rootlets in Damonia 

 take a curved course caudoventrad to emerge on the ventral 

 periphery of the medulla, exhibiting in transverse section much 

 of the appearance of the hypoglossal roots of higher forms (fig. 1). 

 The foregoing description is one generally applicable to the XII 

 root of reptiles, since in these forms the amount of white reticular 

 substance incorporated in the floor of the medulla is greater than 

 that characteristic of the Ichthyopsida (Kappers, 32). 



The differentiation of a hypoglossal cell group in the rostral 

 end of the cervical motor column is not so evident in Chelone 

 as in Damonia. The same may be said with regard to this 

 region in Testudo (Lubosch, 34), Alligator, Varanus, and espe- 

 cially Boa (Kappers, 32), though de Lange has observed a more 

 highly specialized XII cell arrangement in Chamaeleon (Kappers, 

 33, p. 42) and Gisi has described a definite but small-celled XII 

 nucleus in Hatteria (20, p. 180). 



With regard to the incomplete separation of the hypoglossal 

 nucleus from the rostral cervical motor column in reptiles, it is 

 of interest to note that Willard has found a constant admixture 

 of cervical and hypoglossal fibers in the last hypoglossal root 

 in Anolis, though the more rostral rootlets of this nerve were 

 composed entirely of hypoglossal fibers (43, p. 88). In Varanus, 

 on the other hand, according to Watkinson (42, p. 467), the 

 cranial element of the hypogossal nerve which results from the 

 union of three XII rootlets is only joined by the cervical elements 

 outside the cranium. 



Nerve X 



In Damonia the vagus nucleus is dorsally situated, lying for 

 the most part in the periependymal gray matter ventrolateral 

 to the sulcus limitans (fig. 2) . Rostrally it begins a short distance 

 in front of the first X motor rootlet, and extends caudally to the 



