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Mr. E. W. L. Holt. Some Stages in the [Feb. 20, 



by the walls of tlie infundibnlum. These break down above the body, 

 except for a fine celhilar membrane. Behind the body is seen the 

 tapering, downwardly-bent anterior end of the notochord. 



In the early post-larval stage * " an apparent rectification of the 

 cranial axis " has taken place, by the upward rotation of the cere- 

 brum on its posterior end, doubtless owing to the rapid development 

 of the oral and trabecular cartilages, and consequent forward rotation 

 of the mouth. The same causes have also operated so as to with- 

 draw the diverticulum of the 3rd ventricle from its position below 

 the optic commissure. Changes are noticed in the arrangement of 

 the nervous tissues. The olfactory nerves are longer, and the nasal 

 sacs further from the brain. The fibrous bridge over the 3rd ventricle 

 (behind the pineal body), terminating with the posterior commissure, 

 is well marked. The tips of the tectum lobi optici are seen above it. 

 The bases of the optic thalami (walls of the thalamencephalon) have 

 increased in breadth. The mid-brain is comparatively large, and the 

 optic ventricles much more advanced. The cerebellar fold rapidly 

 thins out in the middle, assuming the form, in transverse section, of 

 thick lateral elements united by a cellular band. The infundibulum 

 has undergone vertical flattening. The future lobi inferiores are 

 indicated as lateral expansions, behind which the 3rd oculomotor 

 nerves pass outwards from the centre of the ventral surface of the 

 cerebral mass. The infundibulum extends some way back above the 

 notochord as a thin- walled sac. Its walls are little plicated compared 

 with those in some other forms, e.g., Bhombus,f Anarrhicas .% 



In the J-inch stage the olfactory lobes appear as bulbous masses 

 projecting from the front end of the cerebrum. Fibres can be traced 

 from the optic nerves up to the fore part of the optic lobes. A pale 

 median septum appears between the anterior extremities of the 

 lateral optic ventricles, its base resting on the fibrous tract over the 

 hind part of the 3rd ventricle. The tip of the valvula now appears 

 in transverse section before its connexion with the cerebral mass 

 can be made out, having thus grown forward. The cerebellum has 

 greatly increased in size : instead of terminating as before on the 

 surface of the brain, it is now continued into a thick fold bent 

 sharply down on the anterior portion ; its posterior end passes at 

 once into the thin roof of the 4tb ventricle. Two fibrous bands cross 

 over the aqueduct of Sylvius in the substance of the cerebellum ; their 

 lateral extremities are fused. The lobi inferiores are better marked 

 than in earlier stages. Longitudinal bands of fibres pass back from 

 the roots of the oculomotor nerves through the medulla oblongata. 

 Groups of large ganglionic cells appear on either side of these bands, 



* Balfour, ' Development of Elasmobranch Fishes.' 

 t Stieda, ' Zeitschr. Wiss. Zool.,' 1869. 

 % Mcintosh and Prince, op. cit. 



