20 Dr Gaskell, The Origin of Vertebrates. [Nov. 25, 



side and the pallium becomes membranous; the cerebellar hemi- 

 spheres are seen in Elasmobranchs to be formed by the invasion of 

 nervous matter over the membranous folds on each side of the 

 median lobe or worm, and in the Amphibians the whole cerebellum 

 is reduced to a small band of nervous matter accompanying the 

 IVth nerve across the dorsal part of the membranous tube ; finally 

 in Ammoccetes the whole of the dorsal part of the mid and hind- 

 brain is formed by large membranous folds with the nervous 

 system lying ventral to this membranous tube ; at one place only 

 is there any crossing of nervous matter to the dorsal side, viz. at 

 the IVth nerve and commencing cerebellum. We see then that 

 the phylogeny of the vertebrate central nervous system shows a 

 distinct tendency towards the separation of the nervous system 

 into two parts, viz. a non-nervous epithelial tube and a purely 

 nervous part ; follow that tendency one step further than the 

 stage reached in Ammoccetes, separate the two entirely and we 

 find immediately a nervous system placed in the same position 

 with respect to this epithelial tube as the nervous system of an 

 Arthropod with respect to the epithelial tube of its alimentary canal ; 

 we see moreover that in the lowest Vertebrate, the Ammoccetes, 

 the old mouth and oesophagus, the old cephalic stomach and 

 straight intestine are most easily seen and recognised. Further, 

 in absolute accordance with this gradual and perfect evolution, 

 is the history of the nervous system apart from its tube, for just 

 as parts like cerebellum and cerebral hemispheres diminish in size 

 as we descend, so other parts which are insignificant in the highest 

 Vertebrates come out more and more conspicuously as we descend. 

 This is especially true of the ganglia habenuke and pineal eye, and 

 we see that in Ammoccetes the pineal eye is most conspicuous, is 

 arthropod in type and that the large right ganglion habenulce is 

 its optic ganglion. In fact, as already shown in previous papers, 

 the central nervous system of Ammoccetes fits in both topo- 

 graphically, histologically and physiologically with the central 

 nervous system of an invertebrate such as Limulus or Scorpio. 



Embryologically the phylogenetic history of the old alimentary 

 tube is represented by the formation of the medullary canal with 

 its external anal opening by means of the neurenteric canal ; 

 while the history of the formation and growth of the nervous 

 system round this canal is illustrated by the formation of the 

 cerebral vesicles, the formation of the posterior fissure, and of 

 the substantia gelatinosa Rolandi and of the raphe. 



Again, trace downward the formation of the cavity of the 

 cranium and spinal cord. In the highest Vertebrate a cranium so 

 filled with nervous matter as to leave its impress on the confining 

 walls and a spinal cord surrounded by membranes containing the 

 cerebro-spinal fluid. As we descend, more and more room in the 



