Developmental history of primary segmeuts of the vertebrate head. 425 



confined to the neural axis, but as Locy pointed out, it involves the 

 rest of the germ layers, and the question of primary segments is thus 

 wider than that of metamerism of the head. It is a general embryonic 

 segmentation, most readily seen in the neural axis which is the seat 

 of most active growth in these early stages. 



In describing neural segments there has been more or less con- 

 fusion and for purposes of clearness it becomes necessary to inquire 

 into the means of identifying segments. In the preceding pages the 

 term "neural segment" has been used because it is purely descriptive 

 and does not imply any morphological interpretation. It simply 

 recognizes the fact that the neural axis is divided into a series of 

 joints or segments. 



As boundaries of encephalic segments in trout and chick embryos, 

 I have used external and corresponding internal transverse con- 

 strictions or grooves. These constrictions are uniform in number and 

 position in early embryonic stages, and divide the encephalon of the 

 neural axis into what I believe to be morphologically identical seg- 

 ments or joints. In the early stages these grooves encircle the en- 

 cephalon but in later stages the primary segmentation is confined only 

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

 appearance in the dorsal region of a thin roof. It should be remem- 

 bered 1) that in the position occupied by the 3rd, 5th and 6th seg- 

 mental grooves, deep external constrictions appear that form the 

 posterior limits respectively of the fore-brain, mid-brain and cerebellum, 

 and 2) that all the primitive grooves vanish during embryonic growth, 

 those of the fore-brain first and those of the mid-brain second and 

 lastly those of the hind-brain. In the chick when 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 (see Fig. 45 g\ f). 



Much importance has been attached to the definition given by 

 Orr of a typical neuromere (McClure, Waters, Neal and others). 

 It will be of advantage to make a few observations upon the morpho- 

 logical value of the factors used in this definition. I have observed 

 all five characteristics considered by Orr as fundamental. Regarding 

 the 1st, viz., that "each neuromere is separated from its neighbor by 

 an external dorso-ventral constriction and opposite this an internal 

 sharp dorso-ventral ridge", it should be remembered that the "dorso- 

 ventral ridge" is absent in Teleosts, and in the chick it has at its 



28* 



