THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 6 1 



resemblance to that of the vertebrate. In the former case a trans- 

 formation also takes place, a marvellous transformation, characterized 

 by two most striking facts. On the one hand, the resulting animal 

 is more like a higher vertebrate, for, by the formation of new 

 cartilages, its cranial skeleton is now comparable with that of the 

 higher forms, and the beginnings of the spinal vertebrae appear ; by 

 the increased formation of nervous material, its brain increases in 

 size and complexity, so as to compare more closely with higher 

 vertebrate brains ; its eyes become functional, and its branchiae are 

 so modified, simultaneously with the formation of the new alimentary 

 canal in the cranial region, that they now surround branchial pouches 

 which are directly comparable to those of higher vertebrates. On 

 the other hand, the transformation process is equally characterized 

 by the throwing off of tissues and organs, one and all of which are 

 comparable in structure and function with corresponding structures in 

 the Arthropoda — the thyroid of the Ammoccetes, the tentacles, the 

 muco-cartilage, the tubular muscles, all these structures, so striking 

 in the Ammoccetes stage, are got rid of at transformation. Here is 

 the true clue. Here, in the throwing off of invertebrate characters, 

 and the taking on of a higher vertebrate form, especially a higher 

 brain, not a lower one, Petromyzon proclaims as clearly as is possible 

 that it is not a degenerate elasmobranch, but that it has arisen from 

 Ammocoetes-like ancestors, even though Myxine, Amphioxus, and 

 the tunicates be all stages on the downward grade from those same 

 Ammoccetes-like ancestors. 



As to the eyes, they are functional in the adult form and as service- 

 able as in any fish. There is no sign of degeneracy; it is only possible 

 to speak of a retarded development which lasts through the larval stage. 



Comparison of Brain of Ammocqites with that of an 



Arthropod, 



Seeing that the steady progress of the development of the central 

 nervous system is the most important factor in the evolution of 

 animals, it follows that of all organs of the body, the central nervous 

 system must be most easily comparable with that of the supposed 

 ancestor. I will, therefore, start by comparing the brain of 

 Ammocoetes with that of arthropods, especially of Limulus and of 

 the scorpion-group. 



