No. 2.] DEVELOPMENT OF PETROMYZON: 277 



the olfactory lobes or the ganglia. I cannot agree with Ship- 

 ley in considering this tissue to be nervous, for, when examined 

 with high powers and strong illumination, the constituent cells 

 are seen to be polygonal epithelium cells, very different in 

 character from the rounded ganglion cells found in the brain at 

 this stage. They would seem to be simply a part of the thick- 

 ened epithelium of the nasal involution. But be this as it may, 

 these cells cannot give rise to either the olfactory lobes or gan- 

 glia, for they remain unchanged in position and appearance 

 long after the formation of the olfactory lobes and nerves, and 

 are included between these structures as a single mass. The 

 ganglia must, of course, arise at the distal ends of the olfactory 

 nerves. 



2. The Optic Nerves. — These are peculiar for the very 

 large proportion of the primary optic vesicle which goes to 

 their formation. In other vertebrates the retina takes up 

 nearly the whole of the primary vesicle, while the nerves are at 

 first very short and grow out at a later stage, but in the lam- 

 prey the retina is at first exceedingly minute (Figs. 24 and 26, 

 PI. X) and the optic nerves correspondingly long. The cells 

 of the hollow stalk thicken and obliterate the lumen, and then 

 begin to develop fibres (Fig. 26), a transformation which is 

 completed either before or shortly after the time of hatching. 

 Throughout larval life the optic nerve remains very slender and 

 inconspicuous, in accordance with the undeveloped condition of 

 the eye; but at the period of metamorphosis it becomes much 

 stouter, when the eye reaches its final stage of completion. 



3. The Ociilo-Motor Nerves. — Shipley states that the eye- 

 muscle nerves are not developed till a late stage ; a statement 

 which is very probable in view of the rudimentary character of the 

 eye-muscles during larval life. I have not succeeded in finding 

 any trace of the fourth and sixth pairs in the larvae, and, unless 

 the ciliary ganglion be regarded as belonging to it (iii.. Fig. 41, 

 PI. XI), none of the third pair. This ganglion appears early, 

 though I cannot describe its mode of origin ; it lies above and 

 somewhat internally to the ophthalmic branch of the fifth. 



5. Tlie Trigeminal Nerves. — These nerves are very con- 

 spicuous objects in the later embryonic stages, where the large 

 Gasserian ganglia bulge out the side walls of the head (Figs. 23 

 and 24, PI. X). According to Shipley the ganghon is derived 



