44 NERVOUS SYSTEM OF VERTEBRATES. 



require much more space than .can be devoted to it. The forebrain 

 is formed by the relatively enormous growth of the first neuro- 

 mere, especially of its dorsal part. The optic vesicle, which will 

 be treated more fully later (Chapter VIII), is formed from the 

 dorsal part of the second neuromere. In the ventral portion of 

 the second neuromere there is a great expansion which results 

 in the formation of the inferior lobes and mammillary bodies. The 

 floor of the second neuromere is depressed just behind the optic 

 stalk and expanded backward beneath the third neuromere. The 

 extreme ventro-caudal portion of the inferior lobes thus formed 

 becomes thin-walled and is supplied with a great profusion of 

 blood spaces. This is the saccus vasculosus which becomes 

 closely related with the hypophysis in the pituitary body. The 

 slight constrictions between neuromeres i and ii, and ii and iii 

 quite disappear, while that between neuromeres iii and iv remains 

 and is deepened. The formation of the parietal flexure is largely 

 due to the great growth of the dorsal part of neuromeres iv and 

 v, where the optic lobes are to be formed. The whole of neuro- 

 meres iv and v fuses into the mesencephalon and the constriction 

 between neuromeres v and vi is deepened more than any other. 

 This marks the boundary between the midbrain and cerebellum and 

 is known in the adult as the isthmus. Neuromere vi forms the 

 metencephalon and the dorsal part of it becomes greatly enlarged 

 in many vertebrates as the cerebellum. The neuromeres from 

 number vii caudally enter into the medulla oblongata and spinal 

 cord, the exact number belonging to the medulla being uncertain, 

 since no definite limit can be fixed between brain and spinal cord. 

 It is possible that this limit varies in different vertebrates. As the 

 result of these changes, the brain loses its primitive segementation 

 and comes to be divided into five secondary segments of unequal 

 size. 



The optic vesicle has been mentioned as formed from the dorsal 

 part of the second neuromere, the first of the two which constitute 

 the diencephalon. At a very early stage while the nervous system 

 is still in the form of a flat neural plate, the area which is destined 

 to enter into the retina may sometimes be recognized in the broad 

 cephalic part. The two retinal areas lie near the lateral borders 



