222 HOWARD A VERS. [Vol. IV. 



he was unable to take into account any of the higher sense 

 organs. Of course their relation to locomotion and the central 

 mechanism effecting this relation entirely escaped him. 



I hope to show that there are certain ganglionic centres in 

 the anterior end of the nerve cord of Amphioxus, which compel 

 us to call this region a brain strictly comparable with that of 

 other vertebrates ; for in this region we have the centres of 

 special sense brought into relation with locomotion. 



In my final paper I shall give an historical review of the ideas 

 that have been held concerning vertebrate cephalogenesis. For 

 it is in the gradual evolution of these ideas that we have the 

 firmest support, and completest precedent, for leaving the pre- 

 vailing ideas for those that can be shown to come nearer the 

 truth in nature. The idea that the human skull (as typical of 

 the highest vertebrates and hence erroneously thought to most 

 certainly present genuine vertebrate conditions) was formed of 

 relatively slightly modified vertebral bodies and their processess, 

 had its eminent exponents and ran its course. It was succeeded 

 by the theory that it was only in the cartilaginous cranium that 

 remnants of primitive segments were to be sought. This form 

 of solution was modified by the statement that only a portion of 

 the primordial cranium could possibly show traces of the primi- 

 tive segmented character. 



This brings me to the expression of the view, which has firm 

 support in facts, that the primordial cranium is never influenced 

 by vertebral segmentation, as it appears in, and belongs to, a 

 stage antecedent to the formation of vertebrae ; further, that it 

 is not primarily influenced by the mesomeric segmentation of 

 the body, since it arose at a later period phylogenetically and 

 after the mesomery ontogenetically. The primordial cranium 

 has been gradually acquired by vertebrates, and its rudiments 

 were developed first in or near the horizontal plane in which the 

 chorda lies. Extending in a direction more or less parallel to 

 the chorda, it does not necessarily show traces of the primitive 

 segmentation of that part of the body in which it is developed, 

 since many, at least, of the features of segmentation, had long 

 vanished before a protecting cranium formed about the nervous 

 axis. In such a structure we would expect coenogenetic (onto- 

 genetic) variations to be of frequent occurrence. 



What I have said above does not exclude the possibility of 



