The Brain of Diemydylus Viridescens 289 



connection of the pineal eye with the brain. He says (p. 282) 

 that some of his sections seem to indicate that an opening 

 exists between the second vesicle and the brain cavity. Gas- 

 kell (19, p. 433) considers that the left habena serves as the 

 nerve for this sac and the right as a nerve for the epiphysis. 



From the present investigation no indication of the opening 

 of the cavity of the epiphysis into the sac lying ventrad of it 

 is found, in fact the two are separated by a connective tissue 

 cushion and a blood vessel (Fig. 104, 105). Nor is there in- 

 dication that the habense serve as nerves for either the epi- 

 physis or this sac, though there may be a correlation of their 

 unequal size with the comparatively developed condition of 

 the epiphysis. The left habena (Fig. 105) extends under the 

 epiphysis. At the right is seen a small cavity opening into 

 the general cavity. This is the second vesicle and probably 

 the opening which Alhborn considered as possible. Figure 

 104 shows a more cephalic section in which the tip of the left 

 habena, covered by endyma, extends into the same cavity 

 but maintains its lateral position with regard to it. The re- 

 construction of the cavity is shown in figure 103, where it is 

 represented upon the meson though its opening is not 

 exactly at the middle of the section. This is because an 

 organ, the left habena, which is admitted to be morphologi- 

 cally a lateral organ, has, from unequal growth, assumed a 

 mesal position and pushed aside a small mesal structure. 



From the relation of this second vesicle to the habena, and the 

 supracommissure, from its morphologically mesal position, and 

 its relation to a blood vessel dorsad of it, I conclude that it is 

 the paraphysis, even though no plexuses in the brain serve 

 further to determine its identity. This is in consonance with 

 the statement of Scott (46) that in the earlier stages of petro- 

 myzon, the two dorsal vesicles are soon pushed to the left of 

 the meson ; and of Goronowitsch (21) that in Acipenser ruth- 

 emis the habenae and dorsal sac are asymmetrical. 



In the adult of mammals the remains of the second and 

 more cephalic of the two mesal outgrowths observed in the 

 embryo has not been identified. The caudal is the cona- 

 rium. In man and very markedly in the sheep, as shown by 



