BRAIN OF ONE OF THE SALAMANDERS I03 



indicate that the origin of these commissures in this form is essentially 

 the same as in higher vertebrates. In an embryological study, the 

 course of the fibers with reference to the interventricular foramen must 

 be considered. A comparative study of embryos may show that 

 the transition from one condition to the other does not involve the 

 supposed difficulties. To bring about such a change of relations we 

 do not need to consider the fibers of the bundle as migrating across the 

 foramen. The place of appearance of the anlage of these commissures 

 varies somewhat in different forms. In birds and mammals it is in 

 front of the foramen, while from Mrs. Gage's figures, already referred 

 to, it appears that in Amphibia it is below the foramen. Now, it seems 

 reasonable to suppose that in one case the fibers arising in a given part 

 of the anlage, grow across above the ventricle and in front of the fora- 

 men and in the other case below the ventricle and behind the foramen. 

 Thus the change of conditions might easily come about without the 

 migration of the bundle as such across the foramen. To determine 

 whether this is actually the case and clearly to identify the anlage of 

 the "callosum" of Amphibia with its homologue in higher forms are 

 questions for comparative embryology. 



Diencephalon. — There is no marked line of separation between this 

 segment and the mesencephalon. On the dorsal surface of this segment, 

 at its forward extremity, appears the choroid plexus, which dips down 

 and shuts off the cavity of the mesencephalon from the longitudinal 

 fissure. In its folds lies the paraphysis. Just back of this are the 

 habenulae and supracommissure, and just back of this the pineal body. 

 The conspicuous features on the ventral surface are the large infundi- 

 bulum and the hypophysis. The optic chiasma, at the forward Hmit 

 of the diencephalon, lies sunken in the surface and makes no promi- 

 nence. In this segment the cavity of the brain has its greatest dorso- 

 ventral expansion but is of narrow lateral dimensions. Just in front 

 of the chiasma, on either side, it expands into a narrow pocket, known 

 as the preoptic recess, representing the primitive lumen of the optic 

 vesicle. The ventricle sends off a ventral spur which enters the infun- 

 dibulum by a narrow passage and then expands laterally to a consider- 

 able width. The dorsal and posterior wall of the infundibulum is 



