262 SCOTT. " [Vol. T. 



described it as the commissura tenuissima; Osborn (27) has 

 found it in several of the Amphibia, and named it the Superior 

 Commissure. It also seems to occur in the Selachians, as 

 shown by Balfour. My own observations on the formation of 

 the ganglia habenulae agree entirely with those of Shipley. The 

 asymmetrical development of those bodies is even more de- 

 cidedly marked at the period of their origin than in the adult, 

 and is carried so far that a deceptive appearance of symmetry 

 is produced, for the left ganglion is so small that unless one's 

 attention is particularly directed to it, it may easily be over- 

 looked, and in slightly oblique sections the right ganglion 

 occupies so much of the roof that it appears like a median 

 unpaired structure. The history of these two bodies is very 

 different, and they must be described separately. The right 

 one, as already stated, is much the larger from its first appear- 

 ance ; it depends from the roof of the third ventricle, occupy- 

 ing most of the breadth of that roof, above the level of which 

 it does not at all project (Fig. lO, PI. VIII). At this stage its 

 broadest part is the base of attachment, gradually tapering to 

 the free end, forming in transverse section a triangle. In young 

 larvae the point of attachment becomes considerably con- 

 stricted and the free portion thus exceeds the base in transverse 

 diameter ; at the same time the point of attachment is shifted 

 more towards the right side. In many sections the body ap- 

 pears to lie isolated within the ventricle, owing to the constric- 

 tion of the neck. In later stages the changes in this body are 

 apparently confined to an increase in size, which occurs rapidly, 

 and especially to a marked increase vertically, so that the 

 body begins to project conspicuously above the roof of the 

 ventricle, and thus forces the pineal stalk over towards the left 

 side. Even the point of attachment of the stalk is to the left 

 of the median line ; but this cannot be regarded as a mere 

 mechanical displacement, but rather as a result of the more 

 rapid and extensive growth of the right-side in this region, the 

 left side lagging behind ; what was formerly the median line 

 thus coming to be at the left side. I have seen nothing which 

 suggests an actual shifting in the point of attachment of the 

 pineal stalk. 



The left ganglion habenulae is at first very minute, consisting 

 of a few ganglion cells on the lower side of the superior com- 



