482 ERNEST W. L. HOLT, 



The pineal body and the fibrous tract roofing over the hind part 

 of the third ventricle are treated at length on page 494 



The white matter, which forms on each side a dorso-lateral stra- 

 tum of the tectum lobi optici, is connected by a narrow tract with 

 that of the optic thalamus. 



The roots of the solid optic nerves i) can at this stage be traced 

 up through the external region of the thalami to the fore part of the 

 tectum. The connection can be more easily followed in the later stages. 

 Below the optic lobes the lateral white matter of the cerebral mass 

 is seen to encroach on the vesicular area surrounding the median 

 fissure or third ventricle, and in succeeding sections (cf. fig. 6), a well- 

 marked transverse commissure (c. d.) crosses this and shuts of part 

 of the ventricle as the lumen of the infundibulum (in.). The flexure 

 of the brain at this stage brings this structure into the same section 

 as the aqueduct of Sylvius (aq.) and cerebellum (cb.), though the latter 

 are properly posterior to it. 



I propose to term it the dividing commissure (commissura dividens), 

 as it may be said, at its anterior extremity, to mark the ventral 

 boundary of the fore- and mid-brains. As far as I know, it has not 

 been noticed in other forms, but it is as well marked in the larval Sprat. 



From its external border fibres extend downwards through the 

 scattered vesicular matter of the posterior portions of the thalami 

 into the great ventral commissure of the infundibulum 2), which lies 

 (figs. 7 and 9 c. i.) immediately behind the optic commissure. A similar 

 ventral commissure is described by Mc Intosh and Prince in Anar- 

 rUchas (6, p. 910, pi. XXIV, fig. 6 f. a.). 



A band of white matter also runs back along the dorsal part of 

 the infundibulum, and disappears anteriorly to the pituitary body. 

 With this exception the posterior wall of the infundibulum is wholly 

 vesicular. 



1) The optic nerves are solid in all the stages of the Herring which 

 I have examined. In this they resemble those of the Goby {^ inch 

 long) and differ from those of the young of AnarrMchas, Labrus, Trigla 

 (cf Mc Intosh and Pkince, 6, p. 909) and Zoarces. 



2) This commissure appears to occupy the same position as the 

 structure, which Oer terms the anterior band in the embryo of Ambly- 

 stoma (op. cit., p. 311). The same observer remarks (p. 321) that his 

 anterior band is identical in position with a „Commissur der basalen 

 Vorderhirnbündel" which, according to Dr. Ludwig Edinger (Unter- 

 suchungen über die vergleichende Anatomie des Gehirns), appears in all 

 classes of Vertebrates. 



