208 bulletin: museum of comparative zoology. 



tion of nerve fibres, but also their distribution in the central nervous 

 system. Gaskell ('89) has rightly insisted that the position of the cell 

 groups which are in connection with the nerve fibres, is the true crite- 

 rion of what forms a nervous metamere, rather tlian the position of the 

 exits of the nerve fibres. The sliifting of nerve roots is too well known 

 to need discussion here. In regard to sensor nerves Miss Piatt ('96) 

 says : " Both development and comparative anatomy tend to show 

 that it is a matter of little moment whether these fibres [of the lateral- 

 line nerves] enter the brain by one nerve root or another." I find as a 

 result of my own studies that the ganglionic cells of cranial nerves enter 

 into fibrillar relation with the neural tube at points quite widely sepa- 

 rated from the encephalomere from which the cells were proliferated, 

 and also that in embryos of different Vertebrates the relations of the 

 fibres of the same nerves to the encephalomeres are variable, not only 

 in the case of ganglionic roots but of medullary roots also, as those of 

 the ti'igeminus, abducens, and glossopharyngeus. lu the swine and the 

 chick the abducens arises from encephalomere VI, whereas in S. acan- 

 thias it is in relation with encephalomere VII. Also in swine and chick 

 the root of the glossopharyngeus is in relation with encephalomere VII, 

 whereas in S. acanthias it passes from the neural tube posterior to this 

 neui'omere. It is obvious, then, that we must take into consideration, 

 particularly in the case of cranial nerves, both the location of the 

 " Kerne " of the medullary roots, and the points or regions of prolifera- 

 tion of the ganglionic cells of ganglionic roots, in order to determine 

 their primitive relationships. 



a. Historical Review. 



An examination of the literature bearing on the question of the re- 

 lation of nerves to neuromeres is rendered difficult by the fact that 

 many investigators have failed to distinguish between medullary and 

 ganglionic nerve relations, and thus have not made clear what they 

 meant by the statement that a nerve " develops " from, or has its 

 " origin " from, the expansion or constriction of a neuromere. The 

 figures of McClure ('90) and of Waters ('92), for example, show a pro- 

 liferation of the ganglionic Anlagen of nei'ves from the neuromeres, but 

 not the relationship of the neuromeres to nerve fibres. "While it seems 

 very probable that the proliferation of ganglionic Anlagen has a bearing 

 on the primitive relationship of the dorsal nerves (sensor portion), our 

 best criteria of the segmental value of encephalomeres, as well as of 

 myelomeres, is their relation to medullary nerves, — i. e. ventral nerves 



