DEVELOPMENT OP PETROMYZON PLUVIATILIS. 205 



shaped. This thin wall I conclude makes the division between 

 the optic thalami and the crura cerebri. 



The hind-brain and mid-brain resemble each other closely 

 in structure, the mid-brain being only a trifle larger. Their 

 cavity, which is at first slit like, becomes triangular by the 

 lateral growth of the roof which pushes the side walls apart 

 dorsally (figs. 40 and 41). This thin roof extends back as far 

 as the second gill-cleft, after which it disappears and the 

 nervous system has the structure represented in fig. 42. 



About the forty-fifth day a median longitudinal fold appears 

 in the thin roof; this is the first of the numerous folds found 

 in the roof of the mid- and hind-brain of the adult (fig. 41). 



The fore-brain still has its thick side walls, the optic thalami. 

 Just in front of the stalk of the pineal gland a commissure of 

 transverse fibres is found which runs from side to side on about 

 the twenty-third day. This commissure corresponds with the 

 Commissura tenuisima, described by Ahlborn in his 

 exhaustive work on the brain of the adult Lamprey. It also 

 probably corresponds with the commissure found by Balfour 

 in Scyllium situated just in front of the base of the pineal 

 gland. Osborn has recently described a similar commissure 

 in the brain of the Amphibia, Menopoma, Meno- 

 branchus, Amphiuma, and Rana, and I have adopted the 

 name he proposes for it, the Superior Commissure. The com- 

 missure of the pineal stalk in the Mammalian brain seems to 

 occupy the same relative position. This superior commissure 

 is at first covered with but a few ganglion cells, but these 

 afterwards increase until two bodies are formed, the Ganglia 

 Habenulse. The left one is very small (fig. 39), but the right is 

 a structure of considerable size, projecting downwards and back- 

 wards, and reducing the lumen of the fore-brain to a Y-shaped 

 slit. These bodies have been fully described by Ahlborn in 

 the adult; it is interesting to note that the curious asymmetry 

 they possess is present from their first appearance. No other 

 commissure has made its appearance by the fifty-second day. 



The cerebral hemispheres show some signs of appearing as 

 lateral outgrowths in my oldest larvae, but no trace of paired 



