437 



crowded together, though the cells of one neuromere do not extend 

 into another neuromere. » 



I find that the above structure is characteristic of the embryonic 

 folds in the medulla of the Newt, Chick and Lizard, in every detail. 



I now give some of my own observations. 



1) I find that the neuromeres of the primary fore brain conform in 

 every respect to the structure described above as characteristic of the 

 hind brain neuromeres. These fore brain neuromeres are prominent 

 in Amhly stoma, in Anolis and in the chick, persisting in the latter till 

 62 hours, or slightly later. 



2) The lateral walls of the spinal cord are divided into neuromeres 

 which while less conspicuous, have all the cellular characteristics seen 

 in the typical neuromeres of the hind brain and in fact are a con- 

 tinuation of the latter. The transition from one into the other is sra- 

 dual, the folds becoming larger anteriorly. 



Now the point I wish to emphasize, is this: 



That the structure of these folds in the lateral walls 

 of the Myelon (which I have called »myelomeres«), conforms 

 in every respect to the four characteristics which are 

 found in the hind brain and primitive fore brain folds of 

 all three forms studied, Conversely, that the structure of the 

 neuromeres of the brain (Encephalomeres) conforms in every 

 essential detail to that of the myelomeres, which goes to prove that 

 the Encephalomeres are not only remnants of segments similar to the 

 myelomeres, but that they were originally continuous, and held similar 

 relations to the mesoblastic somites. I also find that the dorsal roots 

 of the spinal nerves take their origin from the apex of their respective 

 myelomeres in exactly the same manner as the nerves of the medulla 

 do from their respective encephalomeres. 



The most anterior encephalomere is connected with »or gives rise 

 to« the olfactory nerve — this nerve arising from its neuromere ap- 

 parently in exactly the same manner as the hind brain nerves do from 

 their neuromeres, that is from the apex of the ridge. The second ence- 

 phalomere does not give rise to any segmental nerve ; immediately op- 

 posite this is the optic vesicle. I do not, with Orr, consider the mid 

 brain as equivalent to a single encephalomere, but rather, relying upon 

 the observations ofKupffer, as equivalent to two, which have de- 

 generated in the forms above studied but persist in the Teleosts and 

 probably in other fishes. The total number of encephalomeres was thus 

 probably 1 0, these corresponding theoretically to the spaces between 

 the 9 mesoblastic head somites observed by van Wijhe, divided as 

 follows : 



