Observations upon the Development of the Teleosteau Brain. 487 



sides are greatly expanded and the lobi inferiores are somewhat 

 constricted oft from the rest. A band of white matter passes back 

 into the infundibulum (cf. the next stage fig. 16) internal to each 

 lobus (IL). 



Since the last stage the valvula has grown forward so that its 

 anterior end appears in transverse section before its lateral connection 

 with the cerebral mass (cf. fig. 8 f.g.) is visible. The cerebellmn is 

 stout, and is now internally fibrous. The fourth ventricle (fig. 17 v. 4.) 

 is deeper than before, with steep sides. 



Striking changes are now observed in the tissue of the cerebral 

 mass. The 3'^ or oculomotor nerves are more conspicuous than for- 

 merly. They spring from the centre of the floor of the crura, and pass 

 (cf. Ill, fig. 16) outwards between that structure and the infundibulum, 

 their course lying in the grooves (probably caused by their presence) 

 behind the bases of the lobi inferiores (IL). Fibres may be traced 

 upwards from their roots into two longitudinally disposed bands 

 (cf. fig. 16 a /".?.), which pass back, one on each side of the middle 

 line, and are LOt lost sight of till near the end of the medulla ob- 

 longata. 



Groups of very large ganglionic cells and scattered vesicular 

 matter appear amongst the ventral white matter of the cerebral mass. 

 These, towards the commencement of the fourth ventricle (fig. 17 v.4.\ 

 become difterentiated into well-marked, very complex areas {a.g.) of 

 mixed ganglionic, vesicular and fibrous matter, lying externally to the 

 longitudinal fibrous bands (/". ?.). 



The two areas become connected by the very fine transverse com- 

 missure, noticed in the last stage (cf. woodcut-fig. 1 c), which divides 

 the longitudinal bands into dorsal and ventral elements (of which the 

 former shortly disappears) and bridges over the median strand of 

 vesicular matter. At the point where the large auditory nerves are 

 seen passing (fig. 17 VIII) from the upper lateral region of the medulla 

 oblongata, downwards between it and the inner wall of the auditory 

 capsule to the acustic maculae, the central transverse commissure is 

 succeeded by two fibrous bands, which cross each other obliquely 

 without intermingling (fig. 17 c.cr.). They form the centre of a sort 

 of St. Andrews' cross of fibres, the dorsal limbs of the cross passing 

 to the nerve roots, whilst the fibres of the ventral limbs ramify among 

 the complex ganglionic areas (a. g.). 



Thus the auditory apparatus of each side is enervated, at least 

 in part, by the brain tissue of the opposite side. The cerebellum, 



