OcTor.i:R 8, 1896] 



NA TURE 



553 



spheres in ihe dogfish ; by the dorsal growth of nervous material 

 to form ihe optic lobes in the Petromyzon ; by llie occlusion of 

 the ventral part of the tube in the epichordal region to form the 

 raphe, as seen in its commencement in Ammocietes. Finally, 

 evidence of another kind in favour of the tubular formation being 



system of the vertebrate originally consisted of two parts — viz. 

 an epithelial tube and a nervous system outside that tube, uhicli 

 has grown over it more and more, and gives not only no support 

 whatever, but is in direct opposition, to the view that the whole 

 tube was originally nervous, and that the epithelial portions, such 



CH3 



KlG. 1. — Comparison of Vertebrate Brain from Mammalia to Ai 

 (Epithelial parts represented by dotted lines.) 



due to an original non-nervous epithelial tube, is given by the 

 frequent occurrence of cystic tumours, and also by the formation 

 of the sinus rhomboidalis in birds. 



The phylogenetic history of the brain of vertebrates, in fact, is 

 in complete harmony with the theory that the tubular nervous 



as the choroid plexuses and roof of the fourth ventricle, are 

 thinned-down portions of that nerve tube. Passing now to 



2. The anatomical test, we see immediately why this epithelial 

 tube comes out so much more prominently in the lowest verte- 

 brates, for, as can be seen from the diagrams, and is more fully 



NO. 1406, VOL. 54] 



