NERVOUS SYSTEM OF THE VERTEBRATA. 



353 



other Vei-tebrata. The prominences, which do not contain prolongations 

 of the iter, become first visible on the appearance of an oblique transverse 

 furrow, while the anterior pair alone are separated by a longitudinal furrow. 

 In the later stages of development the longitudinal furrow is continued so 

 as to bisect the posterior pair. 



The floor, which is bounded posteriorly by the pons Varolii, becomes the 

 crura cerebri. The corpora geniculata interna also belong to this division 

 of the brain. 



Pore-brain. In its earliest condition the fore-brain forms a single 

 vesicle without a trace of separate divisions, but very early it buds off 

 the optic vesicles, whose history is described with that of the eye. 



Fig. 251. Section through the front 



PART OF THE HEAD OF A LePIDOSTEUS EMBRYO 

 ON THE SEVENTH DAY AFTER IMPREGNATION. 



al. alimentary tract; fh. thalamencepha- 

 lon; I. lens of eye; op.i\ optic vesicle. The 

 mesoblast is not represented. 



Fig. 252. Longitudinal sec- 

 tion THROUGH THE BRAIN OF A 

 YOUNG PrISTIURUS EMBRYO. 



cer. commencement of cerebral 

 hemisphere; pn. pineal gland ; In. 

 infundibulum ; pt. ingrowth of 

 mouth to form the pituitary body ; 

 mb. mid-brain; cb. cerebellum; ch. 

 notochord; al. alimentary tract; 

 laa. artery of mandibular arch. 



The optic vesicles become gradually constricted off from the fore- 

 brain in a direction obliquely backwards and downwards. They remain, 

 however, attached to it at the anterior extremity of the base of the 

 fore-brain (fig. 251 op.v.). While the above changes are taking place 

 in the optic vesicles the anterior part of the fore-braiu becomes pro- 

 longed, and at the same time somewhat dilated. At first there is no 

 sharp boundary between the primitive fore-brain and its anterior pro- 

 longation, but there shortly appears a constriction which passes from 

 above obliquely forwards and downwards. This constriction is shallow 

 at first, but soon becomes much deeper, leaving however the cavities 

 of the two divisions of the fore-brain united ventrally by a somewhat 

 wide canal (fig. 252). 



Of these two divisions the posterior becomes the thalamen- 

 cephalon, while the anterior and larger division (cer) forms the rudi- 

 ment of the cerebral hemispheres and olfactory lobes. For a con- 



B. E. II. 23 



