176 bulletin: museum of comparative zoology. 



ally pass or merge into those of the encephalon, thereby forming a con- 

 tinuous series of constrictions throughout the entire length of the neuron, 

 which increase in size anteriorly." Also, in demonstrations of this con- 

 tinuity, Miss Piatt ('89) stated (for the chick) that " the difference (in 

 size) between the fifth ueui-omere [last neuromere of the medulla] and 

 the next posterior fold is not as great as the difference between the sec- 

 ond and third neuromeres." (Compare Fig. 44, Plate 7.) Locy ('94 

 and '95) says of his neural segments that "those in the trunk, region are 

 continuous with those of the head, and pass into the latter without any 

 transition forms." Zimmermann ('91), on the other hand, does not find 

 the spinal cord in S. acanthias to be segmented. 



While I am able to confirm the evidence of continuity of encephalo- 

 meres and myelomeres as stated by previous investigators, I am unwilling 

 on this ground alone to regard these structures as of the same morpho- 

 logical value. Moreover, it has been shown that the hindbrain neuro- 

 meres and the myelomeres differ both in structure and in development. 



h. .Summary. 

 The evidence presented by the constrictions of the myelon warrants 

 the inference that the existence of the myelomeres is dependent upon 

 the presence of the somites, an explanation by no means possible for the 

 hindbrain neuromeres. The constrictions of the myelon appear only 

 after the somites are formed, and increase in number with the addition of 

 new somites. They are opposite the somites, and are confined to that 

 portion of the neural tube against which the somites lie, i. e. the ventral 

 portion. They present no histological or structural conditions which are 

 not easily reconcilable with the hypothesis of their mechanical formation. 

 In those Vertebrates in which the somites extend farther dorsally with 

 reference to the neural tube, the constrictions of the myelon also have a 

 greater dorsal extent. As soon as the somites lose their rounded form 

 and no longer lie close to the neural tube, the constrictions of the tube 

 disappear. As a whole, the evidence in the spinal region of Squalus 

 fully confirms the explanation given by Minot ("92), viz. that the ap- 

 pearance of the myelomeric constrictions " seems to depend upon the 

 development of the primitive segments of the mesothelium. When the 

 segments are fully formed, and before their inner wall has changed into 

 mesenchymal tissue, they press against the medullary tube and oppose 

 its enlargement ; at least one sees that the tube becomes slightly con- 

 stricted between each pair of segments and slightly enlarged opposite 

 each intersegmental space." Structurally, therefore, myelomeres and 



