262 Journal of Comparative Neurology and Psychology. 



functional divisions of the nervous system and the organs sever- 

 ally innervated by them make the nervous system the best guide 

 to the segmentation of the head. 



3. The anterior end of the head is indicated in existing 

 vertebrates b)' the point at which the neural plate meets the 

 general ectoderm and the entoderm, in those forms in which 

 the rostral entoderm is most completely preserved. Lateral to 

 this the olfactory epithelium is formed. Ventral to it the 

 hypophysis represents the vertebrate paleostoma or invertebrate 

 mouth. That part of the head and brain which extends over 

 the hypophysial opening may be compared to the inverte- 

 brate prostomium and as such contains only cutaneous and 

 olfactory nerves. 



4. It is probable that one or two pairs of gills formerly 

 existed between the paleostoma and the present mouth. This is 

 indicated by the presence of epibranchial placodes and palatine 

 and trabecular arches in Petromyzon, by the mode of formation 

 of the ciliary ganglion, and by the labial cartilages of selachians. 

 Similarly one or two formerly functional myotomes have been 

 lost from the anterior end of the head. The anterior head cav- 

 ity of selachians is the rudiment of one of these. Another 

 more anterior, is possibly indicated by the somatic motor 

 nucleus far forward in the thalamus. The loss of these several 

 organs has been followed by the loss of the typical nerves or 

 their modification into special sense organs. The nerve of Locy 

 and the ciliary ganglion are the only nerves in the first four seg- 

 ments which retain their primitive relations. 



5. Following this region of greatest modification, it is 

 believed that the segments can be reduced to common terms 

 and the segmental position of all the organs determined. See 

 Table B and Figure 10. 



a. The gill slits were originally intersegmental. 



b. Each typical dorsal root contained a general cutaneous 

 component distributed to the skin ; a communis component dis- 

 tributed by way of the posttrematic, pretrematic, and pharyngeal 

 rami to the mucosa; and a visceral motor component to the 

 visceral musculature by way of the posttrematic ramus. The 



