Johnston, Morphology of the Head. 249 



other hand impulses transmitted by the general visceral sensory 

 fibers would be followed by reflex movements of the visceral 

 musculature. Since the visceral fibers passed close to the sup- 

 posed site of the first end buds, their innervation by these fibers 

 is easy to understand. As the end buds became more numer- 

 ous and important they spread into the mouth and over the head 

 and body and the visceral nerves followed them. While it is 

 not possible to explain in detail the arrangement of these nerves, 

 the disposition of such nerves as the ramus recurrens facialis in 

 teleosts is in support of this view. If any relation is to be found 

 between the gustatory and olfactory systems it must rest upon 

 the central relations of the two systems. If the gustatory 

 centers and fiber tracts are so related to the olfactory as to show 

 that the primary olfactory centers belong to the same longi- 

 tudinal zone of the brain as the visceral centers, the close 

 physiological relations between the olfactory and gustatory or- 

 gans have their explanation in their morphological relations. 

 The writer expects that this will prove to be the case and in 

 another section offers some slight suggestions looking in this 

 direction. 



16. The first fo7ir head segments. 



These may be passed over briefly since all of the structures 

 concerned have been treated in the above general discussion. 



a. Segment 4. 



Neuromere iv, anterior head cavity, palatine arch (?), epiphysis 2, N. thala- 

 micus (ciliary ganglion and nerve), lens placode (?). 



The structures peculiar to this segment are all rudimentary. 

 Only the brain is well developed and that chiefly because of its 

 relation to the cutaneous and optic tracts. The dorsal mesoderm 

 is represented by the anterior head cavity in selachians, the 

 lateral mesoderm only by the palatine arches of Kupffer in 

 Petromyzon, if they actually exist. The splanchnic sensory 

 nerve is represented by the rudimentary N. thalamicus from 

 which the ciliary ganglion and nerve are derived; the somatic 

 sensory nerve by the second epiphysis. If the lens is developed 

 from an epibranchial placode it should be placed here in con- 



