Gyrus formation in the cerebral cortex of the ferret. II. Description of the internal histological changes.

The internal changes within a developing gyrus of the ferret cerebral cortex were studied by recording (i) the changing length and direction of the radial tissue lines and (ii) the emergence of the tangential banding of the classical six cortical layers. Together these lines provided a coordinate net whose deformations during development gave an indication of the differential growth occurring within a gyrus. The changes in these features suggested that a gyrus was initiated by an area of local growth appearing in the subplate and then in the suprajacent segment of cortical plate. During subsequent growth there was tangential spreading of the more mature tissue at the gyral crown while at the site of the future sulci the cortical plate remained immature and growth was retarded. During later stages the majority of tangential growth occurred in the parasulcal area. At this site a very much thinner cortex was generated from a segment of cortical plate of the same depth and degree of nuclear crowding as elsewhere, implying that growth here was resolved into tangential spreading. The cells and fibres of the deeper cortical layers of the sulcal cortex eventually became tangentially orientated suggesting that they subserved a commissural function between the columnar systems of adjacent gyri. At the scale prevailing in the ferret, gyrus formation was seen as a configuration which tended to conserve both the total length of the cortical columns and the depth of the individual cortical layers.