148 BIOLOGICAL LECTURES. 



concludes that the eleventh of these coincides with the position 

 of the future vagus. The fate of these beads is followed with 

 some detail, and is regarded as of paramount importance in 

 settling the vexed question of the metamerism of the head. 



It may be that further observations will show that Locy is 

 right, but more evidence is greatly to be desired. That the 

 structures in question really exist is beyond doubt ; that they 

 have the significance which he ascribes to them is far more prob- 

 lematical. In the opinions of others who have studied them 

 the structures are less regular and less symmetrical than 

 described, and their number is not constant. Besides, there 

 are several discrepancies in Locy's figures, which, without 

 explanation, seem to invalidate his conclusions. At present it 

 seems too early to extend the term neuromeres to include these 

 structures. 



It is now nearly ninety years since the question of the 

 segmentation of the head was first broached. Can we say to- 

 day how many segments compose this complicated structure t 

 Several times it has seemed that the answer was close at hand, 

 but as frequently has the investigation of other features thrown 

 doubts upon the previous results ; and to-day, while we can say 

 that there are certainly more than the three or four of Oken 

 and his followers, we cannot say exactly what the number is. 

 Before the answer is placed beyond a doubt, a number of other 

 questions must be solved, not the least of which is the broader 

 problem of the origin of metamerism and the relation of this 

 condition in the vertebrates to that in the lower forms. 



