STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 749 



somites of which they are composed (Plate 36, fig. 30, pr) are 

 very conspicuous in surface views. 



Our sections of this stage are not so complete as could be 

 desired : they shew, however, several points of interest. 



The central canal of the nervous system is large, with well- 

 defined walls, and in hardened specimens is filled with a coagu- 

 lum. It extends nearly to the region of the tail. 



The optic vesicles, which are so conspicuous in surface views, 

 appear in section (Plate 35, fig. 26, op.} as knob-like outgrowths 

 of the fore-brain, and very closely resemble the figures given by 

 Oellacher of these vesicles in Teleostei 1 . 



From the analogy of the previous stage, we are inclined to 

 think that they have a lumen continuous with that of the fore- 

 brain. In our only section through them, however, they are 

 solid, but this is probably due to the section merely passing 

 through them to one side. 



The auditory pits (Plate 35, fig. 27, au.} are now well marked, 

 and have the form of somewhat elongated grooves, the walls of 

 which are formed of a single layer of columnar cells belonging 

 to the nervous layer of the epidermis, and extending inwards so 

 as nearly to touch the brain. 



In an earlier stage it was pointed out that the dorsal part of 

 the medullary keel was different in its structure from the re- 

 mainder, and that it was destined to give rise to the nerves. 

 The process of differentiation is now to a great extent com- 

 pleted, and may best be seen in the auditory region (Plate 35, fig. 

 27, VIII.). In this region there was present during the last stage 

 a great rhomboidal mass of cells at the dorsal region of the brain 

 (Plate 35, fig. 24, VIII. ). In the present stage, this, which is the 

 rudiment of the seventh and auditory nerves, is seen growing 

 down on each side from the roof of the hind-brain, between the 

 brain and the auditory involution, and abutting against the wall 

 of the latter. 



Rudiments of the spinal nerves are also seen at intervals 

 as projections from the dorsal angles of the spinal cord (Plate 

 36, fig. 29, sp.n.}. They extend only for a short distance 

 outwards, gradually tapering off to a point, and situated 



1 "Beitrage zur Entwick. d. Knochenfische," Zeit.f. wiss, Zool. Vol. xxill. 1873, 

 taf. in. tig. IX. 2. 



