92 EMBEYOLOGY OF THE LOWEK VEETEBEATES ch. 



eventually comes to lie right at the anterior end of the hemisphere 

 (Fig. 52, D, 0.6). At the same time the bulb comes to form a 

 definite hollow projection of the brain surface immediately dorsal to 

 the still greatly enlarged olfactory tubercle (p.t). 



Differentiation of the Bkain Eegions in Acanthias. — The 

 development of the brain of Elasmobranchs has been worked out by 

 Kupffer (1906) for Acanthias and his account has been made use of 

 in writing the following short summary. 



Figures of the early stages of the medullary plate as seen in 

 surface. view are given in Chap. XL The medullary plate projects 

 forwards from the posterior boundary of the blastoderm and is raised 

 well above the general surface. As it increases in length its lateral 

 edges become raised up so that the portion on each side slopes inwards 

 and downwards into a kind of valley. Each half of the medullary 

 plate extends back into one of the " caudal lobes " which with growth 

 in length come to project freely beyond the edge of the blastoderm. 



Another result of the increase in length is that the anterior end 

 of the medullary plate comes to project freely forwards over the 

 blastoderm forming a head-fold. Each side of the medullary plate 

 arches inwards towards the mesial plane and the whole becomes 

 converted into a neural tube in a perfectly normal fashion. 



As in the case of Lepidosiren, the first sign of differentiation of 

 the brain into its parts is a division into primitive fore-brain (Arch- 

 encephalon) and hind-brain. The demarcation is again most distinct 

 ventrally where the brain-floor bulges into the ventricle (Fig. 54, B) 

 as a prominent fold. Later on this fold spreads upwards on each 

 side to the dorsal surface forming the rhombo-mesencephalic fissure 

 which marks off the mid-brain from the hind-brain. It is only at a 

 later stage in development that the mesencephalon becomes marked 

 off by a constriction from the anterior portion of the archencephalon 

 which forms the thalamencephalon. 



It is of interest to compare sagittal sections through the brain of 

 the Elasmobranch with the corresponding sections already described 

 for the holoblastic lung-fish. Neglecting small differences in detail 

 there is seen to be a striking difference between the two brains — most 

 marked in the middle stages figured — in relation to the longitudinal 

 axis. In Fig. 54, C the Elasmobranch brain is seen to be as a whole 

 strongly curved in a ventral direction: it shows a high degree of 

 " cerebral flexure." The corresponding stage of the Dipnoan brain 

 is on the other hand almost straight, the superficial appearance of 

 curvature being due mainly to the prominent fold of its floor which 

 projects up into the cavity at the level of the mid-brain. 



This cerebral flexure, which is especially conspicuous not only in 

 the brain of the Elasmobranch but also in the other types of Brain 

 (Mammalian and Avian) that were first investigated development- 

 ally, has played a large part in discussions on brain morphology. 

 Thus the idea, already alluded to, that the tip of the infundibulum is 

 the morphologically anterior end of the brain rests upon it. 



