THE HEAD CAPSULE. in 



structure, formed on the ventral surface of the embryo, and that 

 the original track of the alimentary canal is represented by the 

 ventricles of the brain and the central canal of the spinal cord, 

 which are the persistent remains of a structure corresponding 

 with the alimentary canal in the Arthropoda. 



Although I feel that very great difficulties stand in the way 

 of the complete acceptance of this theory, numerous facts tend 

 strongly towards its verification; and my own observations cer- 

 tainly accord very well with those of the two authors named. 

 These facts were observed by me long before I heard of the 

 views of Patten and Gaskell, and only needed a theory, such 

 as they have put forth, to bring them into accord, and to show 

 that there is a far closer relation between the vertebrate and 

 arthropod types than has been suspected — at least, of late. I 

 therefore avail myself of their views as a working hypothesis. 



Nerve Centres.^ The unexpected discovery of the formation of 

 the primitive archenteron.as a dorsal invagination in the embryo 

 of the larva of the fiy (p. i6), is in some sense a confirmation 

 of the theory that the central canal of the primitive nervous 

 system of vertebrates is the morphological representative of the 

 alimentary tract ; it has, however, yet to be shown that the 

 primitive neuromeres or ganglionic centres are related to this 

 invagination in insects. Hitherto I have not been able to 

 trace any such relationship, but I regard it as far from un- 

 likely that the cells from which the nerve centres arise are 

 primarily derived from the outer surface of the archenteron. 

 In embryos not more advanced than those represented in the 

 figure, I have been unable to detect any trace of nerve centres, 

 but it is possible that they are derived from the invaginated 

 part of the blastoderm, although they first appear on the inner 

 surface of the ventral epiblast, at a later stage. The dark 

 cellular mass which surrounds the archenteron, and extends 

 into the head, is certainly suggestive of some such relation- 

 ship. 



Leydig held that the infra-oesophageal ganglia are the repre- 

 sentatives of the cerebellum and medulla, and it is rather 

 remarkable that Gaskell has not apparently seen that such a 



