PRIMARY NEUROMERES AND HEAD SEGMENTATION 335 



but in later stages the primary segmentation is confined to its 

 base and lateral walls, owing to the neural expansion and the 

 appearance in the dorsal region of a thin roof. He pointed out 

 that in the position occupied by the third, fifth, and sixth seg- 

 mental grooves, deep internal constrictions appear that form the 

 posterior limits respectively of the forebrain, midbrain, and 

 cerebellum; that all the primitive grooves disappear during 

 embryonic growth, those of the forebrain first, those of the mid- 

 brain second, and lastly those of the hindbrain. In the chick, 

 \^■hen the neural folds close to form the neural tube, the walls of 

 the latter expand, not uniformly, but intrasegmentally, and the 

 position of the internal grooves is thus passively elevated upon 

 crests. Contrary to the statement of Locy, he found that in 

 younger embryonic stages of the chick and also of the trout, the 

 histology is very simple, the radial arrangement of cells is absent, 

 the nuclei do not recede intrasegmentally from the inner surface 

 of the brain but are uniformly distributed. In these stages he 

 reports that the only criteria by which he has counted neural 

 segments were external and corresponding internal grooves. 

 Concerning the value of various segmental criteria. Hill stated 

 that mesomeres are found only in elasmobranchs, amphibians, 

 and reptiles, and added that in elasmobranchs, the only group 

 in which their development has been traced, their study has 

 led to a greater divergence of opinion and more conflicting views 

 than is generally supposed. He dismissed branchiomeres with 

 a quotation from IMinot that the gill clefts are not segmental 

 and concluded therefore that the branchial nerves are not in 

 segmental order. He argued that Neal's ('98) conclusions were 

 based on negative evidence and that he had observed the seg- 

 ments where Neal failed to find them. 



Johnston ('05) accepted the number of neuromeres described 

 by Locy as necessary to account for all the nerves and sense 

 organs connected with the brain, and stated that observations, 

 then incomplete, on embryos of Amblystoma punctatum seemed 

 to confirm Locy's work. He contended that the nervous system, 

 acting in the role of a connecting and coordinating system, might 

 well act as a key for the interpretation of the facts secured by a 

 study of the other structures. 



