532 LOCY. [Vol. XI. 



V. General Considerations. 



I. New Aspect of the Problem of Metamerism. 



The neural segments have now been shown to occur in 

 extremely young stages of a number of animals. The fact of 

 their presence in these early stages once established they 

 assume new importance. They have too definite a history to 

 admit of being set aside as mere headings or undulations of no 

 metameric significance. When I first began, some two years 

 ago, to notice these segments in extremely young embryos, I 

 attached no particular significance to them ; but a comprehen- 

 sive study has convinced me of their segmental importance and 

 everything taken into consideration they furnish, I think, a 

 more satisfactory basis for interpretation of metamerism of the 

 head than we have had before. I have pointed them out to 

 several observers, in Amblystoma and the chick, and have 

 found no one who had ever noticed them before ; but through 

 the kindness of others I have had my observations as to the 

 number in the head region confirmed. Since these structures 

 have been overlooked in the earliest stages, it will not be out 

 of place to say a word about their general appearance, and 

 how to see them. 



It is extremely difficult to represent them on paper just as 

 they look. All my drawings are camera tracings, and so far as 

 number and arrangement of the segments are concerned, have 

 been verified and reverified over and over again. They are a 

 little too distinct in some of my drawings ; in making the fig- 

 ures clear enough so there shall be no doubt as to what I mean, 

 their distinctness has been somewhat exaggerated. On PI. 

 XXVI I have given a few photographs from untouched 

 negatives that show these segments more as they appear in 

 the actual specimens. To observe them successfully is largely 

 a question of getting shadows. One can look directly at the 

 crenated margins without seeing the segments at all, but by 

 tilting and rotating the specimens until the proper angle of 

 illumination is found, they may be seen with more or less dis- 

 tinctness, according to the general state of preparation of the 



