Observations upon tlie Development of the Teleostean Brain. 491 



partially constricted anterior groove to the above nerve, whilst its 

 wider posterior portion receives a much stouter nerve, the vagus 

 {Cr. X) which passes down from the dorso-lateral region of the medulla 

 immediately above it. Both nerves pass outwards under the auditory 

 capsule. 



The cerebellum may, for convenience, be divided into an anterior 

 and a posterior portion, the boundary being at the commissural bands 

 noticed in the last stage (cf. woodcut -fig. 3 c.aq.). The anterior 

 region shows no marked change: the bands, however, lie somewhat 

 closer together, and, with the growth of the whole organ, have been 

 carried a short distance back, with the result that their lateral extre- 

 mities come to be forwardly directed. As the general contour of the 

 organ undergoes little or no alteration between this and the next 

 stage it is convenient to refer to the figure of the latter (fig. 18) as 

 illustrating the present condition. It will be seen, by comparing 

 woodcut-fig. 3 with fig. 18, that the dorsal angle of the cerebellum 

 above the commissural bands {c.aq) has been pushed forward from 

 its earlier position by the increase in bulk of the posterior region, 

 whilst a secondary fold has been formed between the angle referred 

 to and the hind wall of the optic ventricle. Another result of this in- 

 crease is the bending down of the posterior on the anterior region, 

 causing a deep vertical fold of the cerebellar floor just behind the 

 commissural bands (fig. 18 c.aq.\ a condition faintly indicated in the 

 earlier stage (cf. woodcut -fig. 3). Three fine transverse bands of 

 fibres appear amongst the vesicular tissue lining the front wall of this 

 fold , the most posterior (and dorsal) of which occupies its apex 

 (fig. 18 c.c). 



The only noteworthy change arrived at in the lyV inch stage is 

 the fusion of the two anterior and earliest bands, which now form 

 a single fairly stout commissure (fig. 18 c.aq.). 



V. The development of the pineal region. 



(Infra-pineal Recess, Superior Commissure, Pineal Gland and associated 



parts.) 



Plate XXIX, figs. 1 and 8 a. Plate XXX, figs. 10, 11, 12, 13 and 14. 

 Plate XXXI, figs. 18—22. 



In the I inch herring the large pineal body (fig. 20 pn.\ to be 

 discussed hereafter, is bent forwards over the roof of the thalamen- 



