ABSTRACT
Tissue was obtained from 14 aborted human fetuses, ranging from 13-32 weeks of gestation (wg). The crown-rump length (CR) ranged from 8.3-33 cm. Frontal sections of the specimens were prosessed for SEM and observation were focused on the areas adjacent to the middle part of the calcarine fissure.At 13 wg (CR 8.3 cm), the visual cortex (area 17) was composed of five zones: viz., the ventricular zone, the subventricular zone, the intermediate zone, the cortical plate and the marginal zone. These five zones showed a series of transformations with increasing age. 1) The ventricular zone became progressively thinner, mitotic activity of the ventrieular cells decreased progressively and finally the ventricular ceils differentiated into a single layer of ependymal cells. 2) The subventricular zone and the inter mediated zone were replaced by fiber bundles of white matter. 3) The cortical plate increased in width, exhibited the greatest growth rate, and became differentiated. At 21 wg (CR 20cm), the lower part of the cortical plate first gave rise to laminae VI and V. At 23 wg (CR 22cm), lamina Ⅳ was established in the middle part of cortical plate. At 26 wg (CR 25cm), laminae Ⅲ and Ⅱ could be identified in the upper part of cortical plate. 4) The marginal zone transformed into lamina Ⅰ at its original site.