194 THE EVOLUTION OF CONTINUITY 



As we have said, a few pages back, we cannot accept 

 Gaskell's theory of the derivation of the vertebrate brain 

 ventricles and spinal canal from an enclosed alimentary 

 tract ; and if our own theory respecting the evolution of 

 brain and cord be consistently applied we are driven to 

 suggest that the ventricles and canal arose as follows : — 



The development in dorsal lateral continuity of the 

 four ancestral ganglionic chains involved their enclosing a 



Fig. 62. — The significance of the plans of the Vertebrate and 

 Invertebrate nervous systems. I. Imaginary serially medu- 

 soid organism with successive nerve -rings surrounding the 

 alimentary tract ; their ganglia being linked together in 

 parallel lines. This may be taken to stand for the inherited 

 nervous potentialities of the developing primitive Invertebrate 

 and Vertebrate alike, n, nerve-rings ; g, ganglia ; A, ali- 

 mentary canal. II. How the plan in I was realised in the 

 primitive Invertebrate, d, dorsal, v, ventral surfaces ; 

 CS compressed anterior segments ; B, primitive brain ; 

 NC, nerve-collar surrounding oesophagus ; vo, ventral 

 ganglia which with their nerves alone represent the nerve- 

 rings of I. III. How the plan in I was realised in the 

 primitive Vertebrate, cn, cranial nerves, representatives 

 of n in I ; sc, spinal cord, dorsally placed, otherwise of 

 similar significance to the ventral chain in II, the nerve-rings 

 being represented by the spinal nerves passing out to right and 

 left. The other letters same as in II. 



dorsally situated ancestral medusoid radial canal-system in 

 all its length as this developed (see Fig. 59, A, upper r). 

 When the usual position of nerve-ganglia in the rim of the 

 medusoid bell is remembered, this supposition does not 

 seem unnatural, and is much less fanciful than that which 

 would make a ventrally situated nervous system grow 

 round and almost obliterate an alimentary tract of long 

 pedigree. The fact that in the developing Amphioxus the 

 neural canal temporarily communicates posteriorly with the 



