THE ORIGIN AND SIGNIFICANCE OF CEREBRAL CORTEX 97 



p. 'iO'^Z; Necturiis, '336, p. 149). There is a web of interlaced branches 

 of ependymal elements and among these a still more densely woven 

 tangle of dendrites and slender, contorted, unmyelinated axons. The 

 thicker descending axons of the dorsal and ventral fascicles of the 

 lateral forebrain bundle are assembled within this neuropil (figs. Ill, 

 113). I have seen similar texture in Golgi impregnations of the head 

 of the caudate in the opossum ('24c?, p. 342), a structure adapted for 

 diffusion and summation of all nervous impulses entering it. 



The urodele type of striatal structure could be transformed into 

 that of mammals by reduction of the olfactory component; further 

 differentiation of the caudate nucleus and amygdala; segregation of 

 the large efferent neurons in the ventral nucleus, which becomes the 

 globus pallidus; and segregation of the smaller elements in the puta- 

 men. In Ambly stoma these changes are only dimly foreshadowed. 

 The ventral fascicles of the lateral forebrain bundle connect with the 

 primary motor field of the ventral thalamus and peduncle and are 

 comparable with those of the ansa lenticularis. The connections of 

 the dorsal nucleus suggest relations with the reptilian neostriatum 

 and the mammalian putamen. Here are probably to be found the 

 earliest indications of those formative agencies which in later phyletic 

 stages led to the differentiation of neopallium. In the still more primi- 

 tive hemisphere of Necturus, these indications were recognized and 

 discussed ('33e). 



The strio-amygdaloid complex as a whole is the highest center of 

 dominance in the control of the skeletal musculature, a role which is 

 enormously enlarged in reptiles and birds. In mammals, parallel with 

 the elaboration of cortex, the part which the striatum plays in the 

 patterning of behavior is progressively reduced, but it retains impor- 

 tant functions of co-ordinating and stabilizing motor performance. 

 Just as the cerebellum is added to the sensori-motor systems for 

 facilitation of muscular co-ordination, so in mammals the striatal 

 complex is interpolated in the efferent cortical systems as an acces- 

 sory facilitating mechanism. ^ 



In the Amphibia we find a critical stage in the morphogenesis of 

 the cerebral hemisphere. The definitive major subdivisions are here 

 blocked out in recognizable form, and the pallial part is incompletely 

 segregated from the stem part ; yet the most distinctive feature of the 

 pallium — its superficial cortex — has not yet appeared. We want to 

 know more about the agencies which are in operation here to initiate 

 the separation of pallium from stem and what further changes led to 



