276 bulletin: museum of comparative zoology. 



stages of development of S. acauthias. In the early stages of this 

 animal, the neuromeres are local thickenings of the lateral zones, as well 

 as dilatations of all of the zones of the medulla. As paired glangionic 

 enlargements of the central nervous system, they obviously^ resemble, 

 except in position, the ventral chain of ganglia of Annelids. Therefore 

 they cannot be explained as the passive result of mechanical shoving or 

 bending. The constrictions between the neuromeres, as well as the 

 crowding of nuclei in the regions of constriction, may however be, 

 and most probably are, intensified by shoving or bending of the neural 

 tube. 



No structural conditions are presented by the myelomeres which are 

 not reconcilable with the hypothesis that their existence is dependent 

 upon the presence of the mesodermal somites. If they ever possessed 

 a dorsal segmentation like that of the " hindbrain neuromeres," — and 

 there is no evidence to show that they ever did, — it has been lost. 

 But, though they appear of doubtful morphological value, their numerical 

 correspondence with nerves and somites attests their metameric value. 



The so called neuromeres of the forebrain and midbrain (encepha- 

 lomeres of Zimmermann) are not morphologically comparable with "hind- 

 brain neuromeres," since they are simply dorsal or ventral expansions 

 which are secondary in the time of their appearance. I hold that there 

 ai'e much better reasons — viz. on the grounds of time of appearance, 

 of structure, and of relation to nerves and somites — for regarding each 

 of the primary forebrain and midbrain vesicles (neuromeres I and II) as 

 serially homologous with hindbrain neuromeres (neuromeres III to VII), 

 than for so regarding their later subdivisions. The latter are ccenoge- 

 netic vesiculations of the neural tube, and not of metameric value. 



Both dorsal ganglia and ventral nerves in the trunk develop in the 

 regions of constriction between myelomeres. A comparison with the 

 conditions in Amphioxus and Petromyzon shows that this condition is 

 not to be regarded as primitive, but that previously dorsal and ventral 

 nerves alternated, the former being intersomitic in position. Such 

 topographical relation is retained by some cranial nerves, viz. V, VII, 

 IX, and X (Urvagus). 



The ganglionic Anlagen of four cranial nerves, viz. V, VII, IX, and X, 

 are proliferated from four encephaloraeres, viz. Ill, V, VI, and VII. 

 Chiefly for this reason, but also because of the clear connection of two 

 splanchnic motor roots, viz. V and VII, with two of the encephaloraeres, 

 I conclude that the primitive metameric relations of the latter were with 

 the visceral arches. The local thickenings of the hindbrain neuromeres 



