2 14 Journal of Comparative Neurology and Psychology. 



the root fibers would certainly be influenced to shift backward 

 with it. 



The history of events which have led to the appearance of 

 blank neuromeres in the oblongata, then, is thought to be as 

 follows. The auditory vesicle attained a size in cyclostomes 

 sufificient to push apart Nn. V and VII between which it was 

 situated. Later in the phylogeny there was a migration of the 

 auditory vesicle backward, which was possibly connected with 

 the reduction of myotome 4 and following. This migration, 

 together with a process of concentration of the nerve centers, 

 resulted in carrying all the nerves in question back one segment; 

 Nn. profundus and V following the shifting vesicle for the ad- 

 vantage of approaching their centers, and Nn. VII, IX and X 

 crowded back by the vesicle at the same time that they come 

 nearer their center. The movement of the vesicle was more 

 rapid than that of the roots and the vesicle came to lie between 

 Nn. VII-VIII and IX. Finally, after this position had been 

 assumed, the further expansion of the vesicle crowded N. IX 

 farther back and caused a blank neuromere at this point in rep- 

 tiles, birds and mammals. 



The shifting from the primitive condition which we have 

 supposed may be summarized thus: 



Petromyzon: V not shifted; VII and IX shifted one neuro- 

 mere, crowding upon X. 



Lower gnathostomes: V shifted one, VII, IX and X shifted 

 two neuromeres. 



Reptiles, birds and mammals: V shifted one, VII shifted 

 two, IX and X shifted three neuromeres. 



We may suppose that there was first, in the lower verte- 

 brates a crowding together of IX and X and then a shifting 

 back of both into those neuromeres whose branchial nerves 

 have been collected by N. X (see Sec. 12). 



The blank neuromere was originally the one following the 

 cerebellum and the elements composing the head segment to 

 which this neuromere belonged were: Neuromere vii, somite 3, 

 hyoid arch, somatic motor N. VI, splanchnic motor N. VII, 

 splanchnic sensory N. VII, and a general cutaneous component 



