504 LOCY. [Vol. XI. 



of the epiblast over each branchial cleft. The dorsal branches 

 of certain cranial nerves fuse with these epiblastic thickenings; 

 the superficial part of the thickening giving rise to a branchial 

 sense-organ, while the deeper portion becomes the ganglion of 

 the dorsal root of the cranial nerve. This close relation which 

 exists between the dorsal branches of the cranial nerves and 

 their corresponding sense-organs is undoubtedly of segmental 

 character. But this line of research is beset by a great diffi- 

 culty, namely, that the degeneration of certain branchial 

 sense-organs would, in time, involve the degeneration of their 

 corresponding cranial nerves, and such degeneration has cer- 

 tainly taken place, in part or in whole, leaving in doubt the 

 primitive segments with which they were connected." 



The second and third points mentioned are more important 

 clews to the metamerism of the head. Muscle and nerve are, 

 physiologically, so fundamentally related that we should naturally 

 expect some close correspondence between muscle segments and 

 neural segments, and metamerism of the head region should be 

 studied in light of the work done on both sets of structures. 



The myotomes (or muscle segments) have received by far the 

 most attention as they are the more conspicuous, but it is 

 timely to ask whether they afford the most reliable evidence as 

 to the primitive number of brain segments. Comparative 

 study shows that the neural segments are the first to appear 

 and are less subject to modifications than the muscle segments 

 of the head. The large number of myotomes described in the 

 head of selachian embryos by Dohrn and Killian are more tran- 

 sitory than the neural segments. The period in which they 

 are exhibited is a short one, and soon the seventeen or eighteen 

 segments of Killian, and the eighteen or nineteen of Dohrn, 

 become reduced, by fusion, or absorption, or both, to the nine 

 head segments of Van Wijhe. 



The neural segments, on the other hand, begin very early, 

 as shown in this paper, and preserve their original number and 

 characteristics through several embryonic periods. It will be 

 seen as we proceed in the account of these segments, that the 

 assumption cannot be sustained, that the segmental divisions 

 of the middle germ-layer (protovertebrae) are primitive. 



