No. 3-] THE VERTEBRATE HEAD. 535 



toma in the open neural groove stage and have compared them 

 with hardened specimens of the same age. 



3 . Do they afford a Cleiv to the Metamerism of the Brain ? 



If we grant that these are truly segmental structures, the 

 question still remains : Do they afford the best or even a good 

 clew to the number of segments in the primitive brain ? If so, 

 they must be shown to be equally important in this direction 

 with myotomes, branchiae and cranial nerves. 



In estimating the claims of these various forms of segmental 

 divisions, to rank as primitive, the question of the time of their 

 appearance in developmental history will be significant. On 

 this point I wish to observe, that in all forms I have studied — 

 embracing representatives of Birds, Amphibia, and Selachians 

 — the neural segments are among the first anatomical struc- 

 tures to be established ; before the vestiges of any organs have 

 appeared, the embryo is divided throughout its length into 

 similar segments. These metameric divisions, therefore, ante- 

 date myotomes, branchiae, cranial nerves, or any other struc- 

 tures that exhibit metamerism. They persist through the early 

 stages of development, and become definitely related to seg- 

 mental nerves and segmental sense-organs. In the light of 

 their very early appearance and their history, I think we are 

 justified in saying that they are the most satisfactory traces of 

 primitive metamerism that is presented in the group of Verte- 

 brates. They may be looked upon as a survival of that general 

 segmentation, which all agree in assuming for the (not too 

 remote) ancestral form. 



It should also be observed that the entire embryo is seg- 

 mented, and the term " Metamerism of the Head " should be 

 understood to signify merely regional metamerism and not a 

 different kind of segmental division from that occurring in the 

 rest of the embryo. 



The study of sections shows that the neural segments are 

 distinctly differentiated groups of cells, and not merely a series 

 of undulations. As has been already pointed out the arrange- 

 ment of cells is like that described by Orr for the neuromeres 

 in Anolis. 



