664 BASHFORD DEAN, 



the neural lumen has beeii fully acquired : the sagittal section of the 

 brain at this stage, outlined in Fig. N, corresponds closely to that 

 of a similar stage in Äcipenser, as figured by v. Kupffer ^) (tab. 4, 5, 

 figs. 14, 15). It dilfers from tlie latter mainly in the flatness of the 

 brain wall in the region of the recessus opticus, RO, and in the more 

 down turned position of the lobus olfactorius impar, LO. By this 

 stage in Amia the lobus has already been separated from the for- 

 mative epiblast for (about) 10 hours, relatively at an earlier period, 

 therefore, than in Äcipenser. The sagittal section of the brain of a 

 newly hatched larva of Amia, Fig. O, corresponds with v. Kupffer's 

 tab. 6, flg. 17: its main diöerences include: a greater degree of 

 flexure, a larger lumen anterior to the plica encephali ventralis, PV, 

 a less degree of diflferentiation in the brain's ventral wall and in the 

 commissures. A section of a four days' larva, Fig. P, contrasts with 

 V. Kupffer's tab. 5, fig. 18 as foUows: it possesses a larger lumen 

 in the anterior brain region, an almost obliterated recessus of the 

 lobus olfactorius, a greater thickening of the anterior brain wall im- 

 mediately dorsalward of the lobus, and a greater enlargement of this 

 region, accompanied by the marked thinning away of the paren- 

 cephalon, P, and of the epencephalon, E, in the neighborhood of the 

 velum transversum, VT; also a less degree of differentiation of the 

 cerebellum, A further contrast of the brains of Amia and Äcipenser 

 may be made by reference to Fig. Q, a sagittal section of a larval 

 Amia of four weeks, and a similar section of a larval sturgeon of 

 the same age in v. Kupffer's tab. 8, fig. 19. It will thus be seen 

 that in Amia have taken place by far the more striking changes, and 

 that these have been confined largely to the anterior region of the 

 brain. The anterior two thirds of the roof of the epencephalon (of 

 V. Kupffer) (measured from the lobus olfactorius to the margin of 

 the velum transversum) has thus become greatly thickened, while its 

 posterior third has become ependymatous ; the recessus opticus is now 

 sharply pit-like, the infundibulum narrow and tubulär, its posterior 

 wall elongated, its sacculus ventralis, SV, an almost separate cavity 

 (later of the lobi inferiores). 



Amia diflers, therefore, from Äcipenser in the mode of its brain 

 development mainly in its tendency to ditierentiate the roof of the 

 epencephalon, and to reduce the caliber of the infundibulum, features 



1) Studien zur vergleichenden Entwicklungsgeschichte des Kopfes 

 der Cranioten. Die Entwicklung des Kopfes von Äcipenser, München 1893. 



