DE\-ELOPMENT OF THE NERVES. 73 



hemisphere just below the hippocampal sulcus, which is caused by the ingrowth of 

 the choroid plexus of the lateral ventricle. 



According* to Ecker. the fissure of Sylvius is the first of the primitive sulci to appear. It is 

 visible before the end of the third month, as a wide, shallow depression, which divides the lower 

 margin of the hemisphere into two nearly equal portions, and at the bottom of which is seen 

 the thickening- of the floor of the vesicle from which the corpus striatum and the island of 

 tleil are developed. This fissure appears to be formed by a curving- of the still thin-walled 

 hemisphere vesicle over that thickening-, around which the vesicle bends ; and its anterior and 

 posterior parts ultimately meet along- the line which marks the posterior limb of the 

 Sylvian fissure in the developed brain. The anterior limb is produced much later by a further 

 folding over of that part of the mantle which is in front of the fossa Sylvii. The fissure 

 remains until nearly the end of foetal life as a widely open depression, at the bottom of 

 which the island of Reil is readily visible. It closes gradually from behind forwards. 



The other sulci are distinguished from the four above-numerated in the fact that 

 they are depressions of the surface merely, and not infoldings of the whole thickness 

 of the wall of the hemisphere vesicle. 1 They begin to appear about the end of the 

 fifth month, the fissure of Rolando being the first to show (figs. 80 and 81). 



By the end of the sixth month the precentral and inferior frontal sulci, the 

 intraparietal, the superior occipital, the parallel, the inferior temporal, the calloso- 

 marginal, and the collateral fissures have become visible, as well as the anterior 

 limb of the Sylvian fissure. 



By the end of the seventh month (see fig. 82) most of the remaining principal 

 convolutions and fissures have appeared, and those which were previously present 

 have increased both in length and depth. They are, however, all comparatively 

 short and simple. During the eighth month, they continue to increase in length 

 and depth, and the remaining sulci become gradually developed, but even in the 

 ninth month there are none of the accessory or secondary furrows which add so much 

 to the complexity of the developed brain. The last of the principal sulci to make 

 their appearance are the inferior occipito-temporal. 



DEVELOPMENT OP THE NERVES. 



Spinal nerves. At an early period of development, in some cases even before 

 the closure of the neural groove, in others during or shortly after that event, there 



Fig. 83. TRANSVERSE SECTION THROUGH THE TRUNK OP AN EMBRYO 



SHARK, TO SHOW THE NEURAL CREST. (Ealfour.) 



nc, neural canal ; })r, ganglion rudiment running from neural 

 crest ; x, sub- notochordal rod ; ao, aorta ; sc, \ arietal mesoblast ; sp, 

 visceral mesoblast ; mp, muscle plate ; mp', portion of muscle plate 

 converted into muscle ; J~v, portion of proto-vertelra which wiil give 

 rise to the vertebra ; a/, alimentary canal. 



grows out bi-laterally from the angle of junction of the 

 neural with the general epiblast (fig. 89), and conse- 

 quently at the dorsal aspect of the neural tube a 

 continuous ridge or crest of epiblast, which was first de- 

 scribed by Balfour in elasmobranch fishes : this is termed 

 the neural crest. At intervals along the sides of the 

 neural crest, corresponding with the middle of each 

 mesoblastic somite or proto-vertebra, special clavate 

 enlargements or outgrowths of the neural crest occur 

 (fig. 83, pr\ These grow downwards along the dorso-lateral aspect of the neural canal, 

 between the protovertebra and the canal. They remain for a time attached above 



1 An exception must, however, be made for the collateral fissure, which corresponds with the col- 

 lateral eminence within the ventricle. 



