GENERAL PRINCIPLES OF MORPHOGENESIS 119 



cerebellum, (4) isthmus. In these regions the functionally defined 

 zones are unevenly differentiated, and the cerebellum as a "supra- 

 segmental" apparatus is in process of emergence from the sensoiy 

 and intermediate zones. Similarly, in the forebrain the pallial field 

 exhibits an early prodromal phase leading toward cortical differentia- 

 tion, as seen in reptiles. 



The rhombic brain receives all sensory components of the cranial 

 nerves except the olfactory and the optic. In lower vertebrates, in 

 which the auditory apparatus is at a very low level of differentiation, 

 the sensory components of the rhombic nerves are relatively unspe- 

 cialized, in sharp contrast with the highly specific optic and olfactory 

 systems; and the physiological dominance of the two systems last 

 mentioned in the control of behavior is the determining factor which 

 gives to the cerebrum unique properties that are in marked contrast 

 with those of the rhombencephalon. 



The isthmus is a transitional sector, within which the patterns of 

 all bulbar activities are ordered and integrated. It bulks larger in 

 lower vertebrates than in higher, in which the cerebral cortex has 

 taken over the larger part of this control. Above it the cerebellum 

 was differentiated, not as part of the apparatus which patterns per- 

 formance but as an ancillary mechanism on the efferent side of the 

 arc, to reinforce and regulate the execution of movements. 



Within the cerebrum the two primary centers of dominance — optic 

 and olfactory — are separated by a similar transitional sector in the 

 diencephalon. This is plastic tissue, not dominated by any single 

 sensori-motor system; it is the meeting place of ascending and de- 

 scending sensory paths. In noncorticated vertebrates we find here 

 the apparatus of a type of adjustment from which influences pass 

 forward into the hemispheres and there act as morphogenic agencies 

 in the elaboration of cortical structure. 



It appears, then, that the loci of some characteristic features of the 

 vertebrate brain were fixed by their peripheral connections in the 

 earliest members of the series and that some of these have remained 

 essentially unchanged throughout the phylogenetic history. Others 

 have emerged very gradually from a nonspecific matrix which is dif- 

 fusely spread throughout a wide field. The search for primordia of 

 the latter type in the lower forms as parts of a mosaic pattern with 

 rigidly defined boundaries cannot be successful. In each animal spe- 

 cies the tissue requisite for successful adjustment to the mode of life 

 adopted is fabricated out of such raw material as is available, and 



