[Chronoarchitectonic analysis of the rat brain using radioautography. I. Histogenesis of the telencephalon]

1) Isocortex We find a centrifugal type chronoarchitectonic gradient with, however, a considerable overlap between the various generations of neurons. Thus in the older, deep layers (13.5 days) of the lateral neocortex 35% of the cells are chromologically incongruous (certain are very recent: 17.5 days and more). Cell production is a continuous process and has its peak towards 15.5 days of gestation. This refutes the existence of successive waves of cells. It is not possible to give even an approximate date to the isocortical layers, for the cortical laminations and the isochrones are nor concentric. What finally characterizes this structure from a chronoarchitectonic point of view is the considerable duration of the sequence of neuron production going from 13,5 days to birth. 2) Allocortex We find a centrifugal gradient in the paleocortex. In several of the structures of which it is composed a chronoarchitectonic incongruity can be attributed to the microneurons. It is the case with the islands of CALLEJA (in the olfactive tubercule) which separate very late on. In the archicortex the dominant gradient is again centrifugal. Here we find horizontal variations of the chronoarchitectonic patterns which are discussed with reference to the conception of a possibly fragmented cortex. 3) Striatum the chronoarchitectonic results largely confirm the diencephalic origin of the paleostriatum (STRASSER 1920, SPATZ 1921). In the archistriatum we find a steep rostrocaudal gradient, while the chronoarchitectonic heterogeneity of the structure tends to confirm the conception of a mixed origin (striatum and paleopallium). 4) Claustrum there is a real chronoarchitectonic incongruity between this structure and the isocortex. On one hand the sequence of the anterior part of the claustrum is of the paleostriate type. The posterior part is more probably archistriate. These findings suggest that the formation of the claustrum is of hybrid origin. 5) Septum The characteristic gradient of this formation is caudorostral. This gradient is practically common to all the structures except the telencephalon. We also find a centrifugal gradient related to the presence of a recent "cortex" which could be the counterpart of the cortical septal nuclei in superior mammals.