THE ORGANS OF THE OUTER GERM-LAYER. 



447 



brain and do not cause corresponding projections into the ventricles. 

 Their formation begins as soon as the wall of the brain becomes 

 greatly increased in thickness by the development of white medullary 

 substance, which occurs during and after the fifth month. This 

 is due to the fact that the gray cortex with its ganglionic cells 

 increases more rapidly in superficial extent than the white substance 

 and is therefore raised into folds, the cerebral convolutions or gyri, 

 into which only thin processes of white substance penetrate. At 

 first, therefore, the furrows are quite shallow; they become deeper 

 in proportion as the hemispheres become thicker and the cortical 

 folds project farther out- 

 ward. 



Of the numerous fur- 

 rows which the completely 

 formed brain presents, some 

 appear during the develop- 

 ment earlier, others later. 

 Thus they acquire different 

 values in the architecture 

 of the cerebral surface. 

 For " the earlier a furrow 

 appears the deeper it le- 

 comes, the later it ap- 

 pears the shallower it is " 

 (PANSCH). The first are 

 therefore the more impor- 

 tant and constant ones, and 

 are fittingly to be distin- 

 guished as chief or primary 

 furrows from the subse- 

 quently formed and more variable secondary and tertiary furrows. They 

 begin to appear at the commencement of the sixth month. The 

 first of them to appear is the central furrow (fig. 256 cf), which is 

 one of the most important, since it separates the frontal and parietal 

 lobes from each other. " In the ninth month all of the chief sulci and 

 convolutions are formed, and since at this time the secondary sulci 

 are still wanting, the brain during the ninth month presents a 

 typical illustration of the sulci and convolutions " (MIHALKOVICS). 



Very great differences exist between the different divisions of Mammals in 

 the extent to which the sulci of the cerebrum are developed. On the one hand 

 are the Monotremes, Insectivores, and many Rodents, whose cerebrum also- 



fo 



Fig. 256. Brain of a human embryo at the beginning 

 of the eighth month, after MIHALKOVICS. Three- 

 fourths natural size. 



tif, Central furrow ; veto, hew, anterior and posterior 

 central convolutions ; fo, fissura occipitalis. 



