760 DEVELOPMENT OF THE NEUVOCJS SYSTEM. 



the seventh and eighth months they are developed with gi-eat rapidity, and at 

 the beginning of the last month of intra-uterine life, all the principal ones are 

 marked out. 



The Sf/!r!fi)i _p\\:finv, which afterwards separates the anterior from the middle 

 lobe of each hemisphere, begins as a depression or cleft between them about the 

 fourth month, and, after the great longitudmal, is the first of the fissures to 

 make its ajipearance. It is followed by the fissure of Rolando, and the vertical 

 or parieto-occipital fissure, and somewhat later by the internal fronto-parietal 

 fissui-e. After this, the various subordinate fissures dividing the convolutions 

 gradually make their appearance. By the end of the third month the hemis- 

 pheres have extended so far backwards as to cover the thalami ; at the fourth 

 they reach the corpora quadrigemina ; at the sixth they cover those bodies and 

 great part of the cerebellum, beyond which they project still fiuther backwards 

 by the end of the seventh month. 



During the growth of the hemisphere the aperture of the foramen of Monro 

 is extended backwards ; the arched margin of this opening is curved downwards 

 at its extremities, and forms anteriorly the fornix, and posteriorly the corpus 

 fimbriatum and hippocampus major ; above the margin a part of the wall of each 

 hemisphere comes into contact with its fellow, and in the lower part forais the 

 septum lucidum, while above this the hemispheres are united by the development 

 of the great commissure, the coipus callosum. 



The corpus callo.sum is described by Tiedemann as being first seen about the 

 end of the third month, as a narrow vertical band, extending across between the 

 forepart of the two hemispheres, and subsequently growing backwards. With 

 this view the observations of Schmidt coincide. Reichert, however, maintains 

 that the commissural structure seen at the forepart of the hemispheres is the 

 anterior white commissure, and that the coiiaus callosum appears in its whole 

 extent at once. 



The corpora alhtcantui at fixst form a single mass : so also do the anterior 

 pillars of the fornix, which are distinguished before the posterior pillars. The 

 posterior pillars are not seen until the fourth or fifth month. At that period the 

 hippocampus minor is also discernible. 



DEVELOPMENT OF THE NERVES. 



Spinal Nerves. — Very little is yet known as to the mode of origin 

 of the nerves. In their peripheral extension the great majority of them 

 seem to arise more immediately from mesoblastic formative elements, 

 and the manner in which this takes place has been adverted to in the 

 General Anatomy at p. 161. The ganglia and roots of the spinal nerves 

 are first seen to make their appearance in some very close association with 

 the protovertebral segments. In this the ganglion comes to be distin- 

 guishable as a mass by itself, and the anterior and posterior roots"follow, 

 ■with their jimctiou in the part forming the nerve-trunk beyond the 

 ganglion. But according to recent observations by Balfour in Elasmo- 

 Branch fishes (Scyllium and Torpedo), it would appear that both the 

 anterior and posterior roots may arise iu these animals in a closer con- 

 nection with the nervous centre than was previously believed, and as 

 independent outgrowths from the involuted epiblast of tlie neural 

 canal. The posterior roots are the first to appear, and commence by an 

 outgrowth at the summit (dorsal median groove) of the neural canal, 

 and gradually pass outwards from thence to reach their permanent 

 place of origin in a posterior lateral furrow. A subsequent division of 

 the nerve rudiment takes place into root, ganglion, and a part of the 

 nerve beyond. 



The anterior roots spring by an outgrowth from tlie antero-lateral 

 angles of the cord, one for each muscular plate nearly in the place 



