158 



Journal of Comparative Neurology. 



reader for further grounds for my conclusions concerning the 

 nature of head segments to the more extended paper referred 

 to above. 



,''cam..iT\t.M<i<a. 



ep v<ig 



Fig. 2. Diagram of Selachian head, showing the cephalic metameres and 

 their components, lateral aspect, based upon the study of Squalus acanthias. 

 Upon the basis of the results of Kupffer. Miss Piatt and others a distinction is 

 made in the representation of dorsal nerves between dorso-lateral and medio- 

 lateral (epibranchial) ganglia. 



Secondly, as regards number and composition of cephalic 

 metameres my conclusions have been summarized in Figs, i 

 and 2. There are in vertebrates five pre-otic, one otic and 

 (in Squalus) five post-otic cephalic metameres. The number of 

 post-otic segments whose vertebral components fuse into the 

 occipital region of the cranium of vertebrates is variable. The 

 estimate of the number of pre-otic segments is based chiefly upon 

 the evidences that in this region of Squalus embryos neuromeres 

 and somites numerically correspond, and are in some cases con- 

 nected by motor nerves. For a more extended presentation of this 

 evidence I again refer to the longer paper ('98). Briefly sum- 

 marized, the composition of cephalic metameres from the first 

 to the last is as follows : 



Metamere I. Neiiromere, neuromere I (primary forebrain 

 vesicle) ; dorsal nerve, olfactory (motor component lacking — in 

 correlation with the want of splanchnic musculature) ; ventral 

 netve, absent in correlation with the absence of somatic muscu- 



