Nil. ORGANS OF TUB OUTER GERM-LA 





'.rain and do not cause corresponding projections into the ventricles. 

 Th. ir t'..nii:iii..n IK -ins as soon as the wall of the brain becomes 



In thickness hy tin* development of white modui 

 ubstanoe, whu-h occurs during and after the fifth month. This 

 is due to the far i-ortex with its g.-i cells 



re rapidly in sup. riu-ial extent than the white substance 

 and is then t':v rai> d into folds, the cerebral convolutions or gyri, 

 into which only thin processes of white substance penetrate. At 

 first, theivt'.'iv. tli.- furrows are quite shallow; they become deeper 

 in proportion as the hemispheres become thicker and the cor 



project I'.-irtl 

 \\aitl. 



Of the numerous fur- 

 rows which the completely 

 formed brain presents, some 

 appear during the develop- 

 ment earlier, others later. 

 Thus they acquire diffri 



values in the architect 111'. 



of the cerelral sin; 

 1 'or '//< 



urs the deeper it le- 



'tter it 



pears the shallower it is" 



(PAXSCH). The first are 



mart i/npor- 



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



taut ami WlUtimt ones, and O f the eighth menth, after MIIIAI.KOV us. Three- 



are fdti ,>,,!,, to I, f fourth, natural i. 



J <tf, Central furrow ; vew, *cw, anterior and posterior 



gitisMd </.N- cl, [*>f or prim central oonvolution* ; fo, fljsur* oocipiulia. 



from the subse- 

 quently/' secondary and ' >-s. They 

 begin to appear at the commencement of the sixth month, 

 first of them to appear is the central furrow (fig. 25G cf), win. -h i 

 one of the most import ; 



loh-s in in . ,M-h oiliir. " In the ninth month i and 



convolutions are formed, and since at t! he second 



-till wanting, tin* hrain during tin' ninth month presents a 

 typical illustration of the sulci and coii ( ics). 



Very great difTeivnccs 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 Monotremcs, Insectivores, and many Rodents, whose cerebrum also 



