1890.] Development of the Brain of Clupea harengus. 201 



and are connected by a fine commissure passing through both bands. 

 At the origin of the VIII auditory nerves, tin's commissure is replaced 

 by a St. Andrew's cross of fibres, the dorsal limbs of the cross passing 

 to the nerve roots, and the ventral to the ganglionic areas. 



In the f-inch stage the olfactory lobes are more elongated. The 

 olfactory nerves pass outwards from their anterior extremities. The 

 septum behind the pineal body is larger, and contains a few very 

 large cells. The septum, after losing its ventral connexion with the 

 fibrous tract over the 3rd ventricle, persists for some way back as a 

 cellular leaf-like appendage of the thin median roof of the optic 

 ventricle ; a few fibres pass back into this appendage. 



The optic nerves are longer, from the outward displacement of the 

 eyes. They are very stout and solid, and from their roots fibres may 

 be very easily traced into the optic lobes. Fibres are seen passing 

 from the cerebral mass across the optic ventricle, external to the tori, 

 to the tectum lobi optici. 



The flattening of the brain, noticed by Mcintosh and Prince in the 

 herring of inch, is intensified at this stage, a<nd the brain is also 

 much elongated. 



Large ganglionic cells appear in the tori semicirculares about the 

 region of the splitting off: of the- infundibulum. The white matter of 

 the tectum lobi optici is very conspicuous, and shows traces of three 

 cellular layers in its substance. A circular pale area appears amongst 

 the vesicular matter on either side of the valvula. The lobi pos- 

 teriores are present. Behind them the walls of the medulla approach 

 each other dorsally, shutting off the central canal from the 4th ventricle. 

 The walls recede again further ba<ck before finally closing opposite 

 the last trace of the auditory capsule. 



From behind the region of the auditory nerves a ganglionic area 

 on either side persists backwards through the medulla oblongata. 

 The cerebellum has increased in bulk ; its anterior dorsal angle is 

 carried forward. The fibrous bauds previously noticed are carried 

 further back, and now lie clear of the optic lobes. Three smaller 

 fibrous bands occur behind them. In the herring of ly 1 ^ inch the 

 two first fibrous bands are fused together. 



Pineal Region. 



The roof of the thalamencephalon in the early stages is a single 

 layer of large columnar cells passing forward from the front wall of 

 the pineal stalk. It passes into the roof of the cerebrum, the cells 

 diminishing greatly in size. The superior commissure of Osborn is 

 present from the early post-larval stage ; it is also present in the 

 larval and post-larval Zoarces viviparus, where it is distinctly double. 

 The first signs of the infrapineal recess of Hoffmann are seen in the 



