1 68 Journal of Comparative Neurology. 



the spinal nerves ; that, in fact, the two regions were perfectly 

 homologous in origin, character and function." 



In this connection it will be remembered that Froriep has an- 

 nounced metamerism of a similar sort in the case the mole (two 

 metamers in the diencephalon and three in the mesencephalon.) 



In the recent paper by the same author reviewed elsewhere 

 in this number Froriep withdraws from this position and says that 

 while the position of these folds may be determined by the 

 nerves the origin of the folds is a passive mechanical result of 

 rapid longitudal growth in a limited space. He regards them as 

 of no morphological significance. He concludes: " The joint- 

 ing of the vertebrate body is originally determined by the mid- 

 dle germ layer; where ectodermal structures exhibit segmental 

 arrangement it is a result of secondary adaptation to metamerism 

 of the mesoderm." 



In forming any estimate of the metameric significance of 

 these diverticles as they undoubtedly appear in the fore brain a 

 few fundamental considerations must be kept in mind, (i) The 

 morphological front of the brain cannot be beyond the infundi- 

 bulum. The pituitary is historically closely associated with the 

 neuroporus on the dorsal aspect. The olfactory is allied with 

 the pituitary but is a dorsal organ (Amphioxus, Cyclostomata.) 



(2.) It is scarcely legitimate to count dorsal diverticles like 

 those of the fore-brain with ventral expansions like those of the 

 midbrain and hind-brain. Waters and others seem to have 

 made this mistake. 



(3.) The optic diverticle is not to be -directly compared 

 with the others but is more like a belated portion of the gangli- 

 onic ridge. The morphological entrance of the optic fibers is 

 caudad of the hypophysis-olfactory region. 



(4.) If the olfactory is the first dorsal root the ventral floor 

 of its neuromer is in the mammillary region. 



((5.) The hypoaria and their homologues may represent a 

 second ventral expansion. 



(6.) The formation of the mesencephalic flexure and sad- 

 dle cleft may have played an important part in modifying the 

 originally simple arrangement of the third and fourth nerve 

 roots. 



