68 CLASS PISCES. 



is continued (as the first ventricle) and which give off from their 

 anterior ends the olfactory nerve fibres : they are called the 

 olfactory lobes or the rhineiicephala. In the embryonic brain 

 before the differentiation into thalamencephalon and prosen- 

 cephalon has been effected, the fore-brain becomes bent ventrally, 

 forming an angle with the j^osterior part of the basi-cerebral 

 axis. This bend in the cerebral axis constitutes the cranial 

 flexure ; it takes place, roughly sjieaking, at the junction of the 

 fore-brain and mid-brain and a short distance behind the front 

 end of the notochord, the anterior end of which is under the pos- 

 terior part of the fore-brain. The notochord is therefore in- 

 volved in the cranial flexure and its front end becomes hook- 

 shaped. The other organs of the anterior end of the head 

 are also affected as is shown by the somewhat longi- 

 tudinal disposition of the anterior gill-slits and arches (Fig. 39) 

 as compared with the transverse disposition of those behind. In 

 consequence of this bend in the nerve axis the anterior end of the 

 neural tube becomes eventually directed ventralwards and, by a 

 posterior outgrowth, backwards beneath the floor of the mid- 

 brain : it constitutes the infundibulum, which has already been 

 mentioned as one of the three divisions into which the fore-brain is 

 differentiated. In addition to the infundibulum, a fourth divi- 

 sion of the fore-brain has to be distinguished. At an early em- 

 bryonic stage, before the prosencephalon is marked off, the 

 anterior cerebral vesicle gives off a right and left lateral 

 outgrowth : these are the optic vesicles. They at once become 

 applied externally to the lateral skin of the head, and their con- 

 nection with the brain becomes constricted to form a stalk-like 

 structure which eventually becomes solid and forms the optic 

 nerve. At the same time the cavity of the optic vesicle becomes 

 obliterated by the invagination of its outer wall next the skin 

 upon the inner wall on the brain side. This collapse of the optic 

 vesicle, if not caused by, takes place in connection with the 

 formation of the lens from the outer ectoderm at the point where 

 the optic vesicle before its collapse touched the skin ; it gives 

 rise to the formation of a two-walled optic cup, the mouth of 

 which is occupied by the lens and the double wall of which 

 becomes the retina of the eye. 



To return to the fore-brain. When the cranial flexure is 

 established, the posterior part of its dorsal wall looks forward. 



