26o SCOTT. • [Vol. I. 



important element in the view to be taken of the homologies 

 of the epiphysis. That portion of the pineal stalk which lies 

 beneath the distant vesicle remains, throughout larval life at 

 least, cellular, and retains its lumen. It may be seen as a 

 small thin-walled canal lying between the pineal vesicle and the 

 left ganglion habenulae. The proximal portion of the stalk 

 becomes fibrous, and loses its lumen. The stalk is short in the 

 larvae, but elongates very considerably at the time of meta- 

 morphosis. 



The epiphysis is at first entirely symmetrical and median in 

 position, but afterwards, as has been pointed out by Ahlborn, 

 it assumes a strikingly asymmetrical position, moving to the 

 left of the median line and coming into special relations with 

 the left ganglion habenulae. I have no observations which 

 show the mode of origin of the second epiphysial vesicle, 

 which, according to Ahlborn, is already present in Ammoccetes 

 of 22 mm., nor have I been able to detect anything which 

 suggests the formation of a lens at any stage in the develop- 

 ment. 



In a larva of 26 mm., a series of transverse sections reveals 

 the following condition of this structure. A section passing 

 through the forward portion of the hemispheres shows the 

 anterior edge of the primary pineal vesicle, which is median in 

 position and symmetrical with reference to the hemispheres. In 

 the following section both vesicles of the epiphysis are shown, 

 and here they are distinctly to the left of the median line. In 

 structure the two vesicles are very similar, but the cells of the 

 lower are smaller and less distinctly rod-like than those of the 

 upper. In this section the right ganglion habenulae is seen, 

 but the left is not yet reached. A third section (omitting in- 

 termediate ones) exhibits the hollow pineal stalk resting upon 

 the left ganglion habenulae ; while farther back the stalk be- 

 comes solid and passes into the roof of the recessus pinealis. 

 This is the condition in which the organ remains during larval 

 life, and the only important change which takes place at the 

 time of metamorphosis is the great increase in the length of 

 the pineal stalk, by which the vesicle is carried nearly as far 

 forwards as the lamina terminalis, and entirely in advance of the 

 right ganglion habenulae, and the stalk lies in close contact with 

 the optic thalami, which project above the cerebral hemispheres. 



