No. I.] THE PRIMITIVE VERTEBRATE BRAIN. 45 



The fore-brain neuromcres of the Newt persist up to a 

 certain period and finally disappear, leaving no trace whatever. 

 This we have already found to be the case in the Lizard and 

 Chick. 



Up to this point we have seen that the structure of the 

 folds in the lateral walls of the myelon (myclomeres) 

 conforms in every respect to the four characteris- 

 tics which are found in the hind-brain and primi- 

 tive fore-brain folds of all three forms studied (with 

 one exception in the Newt), which goes to prove that the 

 encephalomeres are not only remnants of neural 

 segments similar to the myelomeres, but that they 

 were originally continuous. 



The mid-brain has been purposely omitted up to this point, 

 but will be considered further on. 



Relation of the Auditory Vesicles to the Neuromcres of the 



Hind-brain. 



The importance of this relationship will be seen further on 

 in connection with the nerves of the hind-brain. The auditory 

 vesicle {and vcs) in the Newt and Lizard is opposite, in a 

 transverse line, to the auditory neuromere {Nm VIII. ; Figs. 4, 

 4^?, 5, and 5^). Li the embryo Chick it holds the same position 

 as in the Newt and Lizard up to the 96th hour, or slightly later; 

 after this its position is shifted backwards to a point between 

 the auditory and glossopharyngeal neuromcres (Figs. 6, 6«, 

 Nm VIII. and Nm IX). 



Relation of the Myclomeres and Encephalomeres to their 



Respective Nerves. 



All the neuromcres of the spinal cord give off (on each side) 

 from their dorsal half a mass of ganglion cells, which constitute 

 the dorsal or sensory roots of the spinal nerve {SpN), Figs, i, 2, 

 and 3. In a like manner I find that four neuromcres in the 

 hind-brain and one in the primary fore-brain give rise to dorsal 

 or sensory roots of cranial nerves. 



The myelomeres on giving rise to the spinal nerves, in the 

 manner stated above, degenerate soon after the nerves are 



