88 PEOF. MARSHALL AND MR. SPENCER. 



Owing to defective supply of materials, our observations on the 

 stages earlier than k are too fragmentary to be relied on ; this we greatly 

 regret, inasmuch as many features in the early stages are of extreme 

 importance, and would well repay thorough investigation. 



In the present paper we propose to confine ourselves to the 

 consideration of the preauditory nerves, reserving the postauditory, 

 which present many features of peculiar interest, for a future occasion. 



Our investigations have been conducted almost exclusively by means 

 of sections of hardened embryos of Scyllium, and, as on former occasions, 

 we have found a mixture of chromic and osmic acids superior to any 

 other hardening agent. For the specimens from which our best 

 preparations have been made we are indebted to the courtesy of the 

 managers of the Southport Aquarium ; our thanks are also due to 

 Mr. A. J. Moss, of Owens College, for his gift of a fine specimen of 

 Mustelus, as well as for valuable assistance in connection with the 

 literature of our subject. 



The Third (Oculomotor) Nerve. — We do not propose to deal in the 

 present paper with either the olfactory or optic nerves, inasmuch as 

 the former has been already fully described, 1 while concerning the 

 latter we have nothing new to communicate ; we therefore commence 

 with the third or oculomotor nerve. 



Concerning the development of this nerve we have very little to add 

 to the account given in the paper already referred to. 2 At stage l it 

 arises from the base of the mid-brain, not far from the mid-ventral 

 line, by a large posterior ganglionic root and by several smaller anterior 

 ones clearly distinguished from the former by possessing no ganglion 

 cells. The nerve itself runs backwards as a long slender stem to the 

 interval between the first and second head cavities (fig. 10, 1 — 2), 

 wdiere it expands into a ganglionic swelling (fig. 10, e.g.) wedged 

 in between the tops of the two cavities. From this ganglion the two 

 main branches of the third arise ; of these the upper one (fig. 15, in b), 

 at a rather later stage, supplies the rectus superior and rectus interims 

 muscles, whilst the lower one (fig. 15, hi c), runs down behind the 

 rectus inferior, and ends in the obliquus inferior muscle (fig. 15, o.i.). 

 At stage K, at which our observations commence, the third nerve has 

 the same point of origin and the same relation to the head cavities ; 



1 Marshall, "Morphology of Vertebrate Olfactory Organ," 'Quart. Journ, Micr. So.,' 

 July, 1879, pp. 300 seq. 

 3 Marshall, loc. git., pp. 78 seq. 



