76 BULLETIN OF THE 



Also caccording to Kohl {'89, p. 407), the nerve-fibre layer, the gan- 

 glion-cell layer, the inner and outer nuclear layers, and the inner re- 

 ticular laj-er are present. Regarding the outer reticular layer and the 

 optic cells he says : " Zvvischeu den beiden Kornerschichten habe ich die 

 ailssere reticulare Schicht (Zwischenkornerschicht) innner durch eine 

 fortlaufende, oft gar nicht so schmale Spalte reprasentirt gefunden. 

 . . . Die Sehzellen, die sich mit Picrocarmin meist sehr schon farben 

 lassen, zeigen ungemein mannigfache Formen : bald ganz flach, bald 

 nahezu kreisrand. Oefter fand ich voUkommen entwickelte Zapfchen, 

 niemals jedoch auch nur annahrend stabchenartige Gebilde. Die Hem- 

 mung in der Entwicklung ist ebeu auch hier schon so friih eingetreten, 

 dass eine ausgesprocheue Stiibchen- und Zapfenschicht nicht mehr zur 

 Ausbildung kommen konnte." 



Of the retina in Talpa, Hess says that the nerve-fibre layer is very 

 thick near the entrance of the optic nerve, and that the inner reticu- 

 lar layer contains cells ; he quotes Leydig and Kadyi to the eftect that 

 the optic cells consist exclusively of rods, and he adds ('89, p. 5), 

 " Ueber die anderen Retiuaschichten ist Besonderes nicht hervorzuhe- 

 ben." Kohl ('89, p. 384), however, states that " Zapfchen sind stets 

 vorhanden : oft vereinzelt, oft sehr zahlreich und die Stahchen nahezu 

 verdrangend. Bei einem Exemplar zeigen die Sehzellen noch jene 

 Form, die sich bei Embryonen eines gewissen Alters findet, und noch 

 nicht erkennen liisst, ob die betrefFenden Zellen sich zu Stabchen oder 

 zu Zapchen weiter entwickeln werden." It thus appears that the three 

 retinas have reached about the same stage in development ; that of 

 Proteus being probably on the whole the most rudimentar}-, and that 

 of Typhlogobius, at any rate as represented by the one shown in 

 Figure 21, the least so. 



As regards the vitreous body, Schlampp finds that it is entirely absent 

 in the eye of Proteus ; while Kohl ('89, pp. 406 and 407) finds a struc- 

 ture which he regards as the hyaloid membrane, or " the membrana 

 limitans interna, the only representative of the vitreous body in the Pro- 

 teus eye." Hess and Kohl both describe the vitreous body as present in 

 Talpa, and, according to the latter, it contains numerous blood-vessels. 

 It will be remembered that no trace of this structure has been found in 

 the eye of Typhlogobius, with possibly a single exception. 



All are agreed at present, it appears, that the optic nerve is present in 

 both Proteus and Talpa, though Hess quotes Semper as stating that it is 

 entirely degenerated in Talpa. I find no account, however, of its ever 

 having in either of these animals a pigment sheath in its passage through 



