434 THE INTERMEDIATE PRIMATES 



limited portions of the nuclear substance. The nucleus in the gibbon shows a 

 number of well-defined plications throughout its entire extent. Its hilus is 

 dorsomesial in position and so disposed as to place its fundus in a ventro- 

 mesial position. The general saccular arrangement of the nuclear substance 

 is most pronounced along the lateral wall. Here also the folia or convolutions 

 are best defined. The mesial wall is much less developed in this respect. It 

 gives the impression of a structure about to emerge from a diffuse matrix, 

 but as yet not possessed of the definition characteristic of ultimate develop- 

 ment. In this sense the dentate nucleus in gibbon represents a transitional 

 stage in which only a portion of it has attained its full evolutional differen- 

 tiation. It affords an illustration of one of the most important movements in 

 the process of unfolding which appears both in ontogenetic and phylogenetic 

 development. The gradual emergence of recognizable features out of a diffuse, 

 more or less indefinite anlage is a rule in the genesis of all organs. The spinal 

 cord, for example, in early fetal stages, shows but little differentiation of its 

 principal histological features. In the early period of human development the 

 dorsal and ventral gray columns are distinguishable but have many points of 

 similarity. Only in infancy do these two columns of gray matter assume dis- 

 tinctive characters. During the first year of life it is often difficult to dis- 

 tinguish histologically between the configuration of the several levels of the 

 spinal cord, even when the comparison involves such widely removed portions 

 as the cervical and sacral segments. Throughout adolescence and into adult 

 life these differential features progressively assume their ultimate character. 

 A similar process passing through gradient stages appears in the phyletic 

 evolution of many structures in the nervous system. Such structures have 

 their inception in a diffuse matrix and gradually acquire sharpness of defini- 

 tion together with specific characteristics. The dentate nucleus in gibbon, 

 therefore, is of great interest morphologically if for no other reason than 

 representing a decisive stage of evolutional transition. Its physiological signif- 



