146 

 teristios. The br-iin of one of the hii^,her vertebc'ites embol- 

 ies many nodi fications of the original plan, viroaght in the 

 course of its graiual evolution. These alterations may even 

 become more conspicuous than the primary structures upon flhich 

 they have been superposed, anl it is only through comparisons 

 with a less differentiated condition that we can hope to dis- 

 tinguish the new from the old. 



A comparative study of the several brain-segments of Wus- 

 telus is productive of some results that might not have been 

 anticipated. The cerebellum is marked by an organization out 

 of all proportion to that of adjacent regions, an organization, 

 moreover, far more highly differentiated than is presented by 

 the cerebellum of either the amphibian or tne reptile. The 

 oblongata, on the contrary, has retained the plan of structure 

 of the primitive neural tube without the intervention of pro- 

 found changes. It is due to this fact that homologies between 

 the oblongata and the spinal cord are so readily traced in this 

 animal. A more extreme degree of simplicity is found in the 

 interbrain, the thalamus having such a slight differentiation 

 as to make comparisons between it and higher thalami somewhat 

 difficult. Finally, the forebrain is far in advance of the 

 forebrains of other fishes. Contrast the membranous pallium 

 of the teleost or the ganoid with the nervous pallium of Muste- 

 lus, which, as has been pointed out in the preceding pages, 

 anticipates the olfactory associations of higher brains to a 

 noteworthy degree. 



