8 JoUKXAI, OF CoMrAKATUK NkIKOLOGY. 



illustrates the nature of the processes to which its formation 

 is due. 



It seems at first sight quite out of the question to expect 

 to find any homologous processes in the brain of the higher 

 vertebrates. In the first place, the relation between the 

 Purkinje cells and the granular zone (molecular layer) is not 

 the same, and, in the second place, there seems to be no 

 evidence that such a wholesale revolution has taken place; 

 but, on the other hand, very sufficient evidence that the 

 increase in surface has been effected by a diff'erent plan, i.e., 

 the corrugation of an organ, which increases in thickness 

 rather than length. It is true that in birds there is an 

 obvious partial revolution of the whole cerebellum through 

 a considerable arc, but this is more easily explained by ref- 

 erence to the flextures incident to the great compactness of 

 the avian brain and the freedom of rotation due to the 

 extensive development of the velum medullare anterior, 

 which leaves the cerebellum quite disassociated from the 

 corpus posterior and free to rotate upon the peduncles as a 

 lateral horizontal axis. 



Among mammalia, the cerebellum of the opossum indi- 

 cates a decided tendency to revolution from behind ceph- 

 alad. Of the several primary convolutions first formed, 

 the posterior continues growth longest and eventually cir- 

 cumscribes the anterior, and, with its secondary gyri, forms 

 the ectal surface, as may be readily seen in longitudinal 

 sections. One additional consideration may, nevertheless, 

 lead to a prosecution of a search for some plan of revolution 

 or evagination in the mammalian cerebellum as well as in 

 the reptilia. It is this: if the conclusions of His and others 

 regarding the typical histogenesis of nervous and connective 

 elements in the nerve-tube be correct, it is difficult to 

 understand the origin of the cells of the cerebellum by ref- 

 erence to the adult relations. We have the ventricular sur- 

 face lined with the usual epithelium, which is, during an 

 early period, proliferous, as may be easily d(?monstrated. 



