276 SCOTT. ' [Vol. I. 



of their very small size, and because in early stages the organs 

 are so closely crowded together, and the tissues so loaded with 

 yolk-granules, that it is often almost impossible to distinguish 

 between them. By long-continued study, however, I have 

 obtained an account, more or less complete, of the development 

 of most of the nerves ; but of some, such as the trochlearis and 

 abducens, I have not been able to observe any trace whatever. 



I. TJie Olfactory Nerves. — The development of the olfac- 

 tory nerves is very obscure, as the nasal epithelium is differen- 

 tiated at an early stage, and lies in immediate contact with the 

 fore-brain, even before the formation of the hemispheres. In 

 other vertebrates there is usually a space between the brain and 

 the nasal pits, which allows the appearance of the nerve to be 

 observed, but in Peti'omyaou there is no such space. When the 

 nerve first appears, I cannot say; but in larvae of about 10 mm. 

 an appearance, perhaps deceptive, of fibres passing from the 

 fore-brain to the olfactory epithelium may be seen. At a later 

 stage (Fig. 11, PI. IX), the nasal pit is somewhat farther 

 removed from the brain, and shows the exceedingly short olfac- 

 tory nerve (in Fig. 1 1 it is cut through obliquely). In larvae 

 of 22 mm. (Fig. 35, PI. X), the nerve is very plainly shown, 

 but the ganglion has not as yet been differentiated. The gan- 

 glia appear at a much later period, and apparently are derived 

 from the olfactory epithelium, though I have not observed the 

 process. With the great increase in size and complexity of 

 the olfactory organ, which takes place at the metamorphosis, the 

 olfactory nerves undergo a corresponding increase (Fig. 22, PI. 

 X). The olfactory ganglia are now very conspicuous bodies, 

 and send off numerous bundles of fibres, which pass along the 

 septa between the chambers. The ganglia are contained within 

 the nasal capsule, and are connected by short nerve-trunks with 

 the olfactory lobes. As far as I have been able to observe, the 

 olfactory nerves are paired from the first, — a fact which has an 

 important bearing upon the question of the primitive or sec- 

 ondary mode of origin of the median nasal organ characteristic 

 of the Cyclostomata. 



Shipley, as already mentioned, has described and figured a 

 mass of cells, which he believes to be nervous, which are at 

 first unconnected with the brain, but continuous with the nasal 

 pit and canal, and which he suggests may give rise to either 



