GROWTH OF THE CEREBRAL CORTEX 539 



are becoming ganglion cells have originated from the germinal 

 cells lying in the ventricular wall and have migrated from there 

 to their final position in the cortex. In the rat brain, cell migra- 

 tion is yet in progress at birth, giving the sections a peculiar 

 aspect. The complication of the cortical lamination in the new- 

 born rat is due to the' existence of one or two transitional layers 

 (so-called 'Uebergangsschichten', fig. 8, Tr.) between the germinal 

 cells (fig. 8, G) in the ventricular wall and the cortex proper. 

 These transitional layers are no more to be seen in the brain 

 weighing more than 0.5 grams (age about 5 days). 



On examining figures 7 and 8 it is seen that the five cortical 

 layers are not yet clearly distinguished. The lam. zon. (fig. 8, 

 /) is fairly thick and the borderline between this layer and the 

 underlying lam. pyr. (Ill) does not run smoothly as in the adult, 

 but shows a fine zigzag, suggesting that the invasion of the zonal 

 region by the pyramids is still going on. The pyramids are not 

 yet in a mature condition, their protoplasm is scanty and homo- 

 geneously stained and their form somewhat spindle-shaped. The 

 nuclei stain less deeply than in the adult brain and the chromatin 

 is not completely visible. The lam. gran. int. cannot be dis- 

 tinguished. The position of the lam. gang. (V) is already sparsely 

 occupied by large-sized pyramids rich in protoplasm, but mixed 

 up with them is a large number of small-sized pyramids, some 

 of which are growing to be ganglion cells and some, probably, on 

 their way to the lam. pyr. The lam. mult. (VI) is divided into 

 two sublayers by a band, poor in cells, as is seen in the adult 

 (fig. 9), but at this phase the number of the ceUs in the ental sub- 

 layer is very much greater and they are larger and better stained 

 than the cells in the ectal sublayer. Their orientation is irregu- 



Fig. 8 Diagram of cell-lamination of the prematurely-born albino rat brain 

 weighing 0.2 gram, schematically enlarged from the designated part (X-X) of 

 figure 7. /, lamina zonalis; III, lamina pyramidalis;'F, lamina ganglionaris; VI, 

 lamina multiformis; Tr., transitional layers; G, germinal layer or matrix at the 

 ventricular wall. Magnification X 100. 



Fig. 9 Diagram of cell lamination of the adult albino rat brain weighing 1.8 

 grams, schematically enlarged from the locality II, figure 2, and corresponding 

 to that selected for figure 8. /, lamina zonalis; //, lamina pyramidalis; ///, 

 lamina granularis interna; V, lamina ganglionaris; VI, lamina multiformis. Mag- 

 nification X 65. 



