170 RICHARD ASSHETON, M.A. 



of a 40 mm. tadpole do not materially differ in shape or character 

 from those of the tadpoles in which nerve-fibres have not yet appeared. 

 I cannot see that there can be the slightest doubt that the nerve- 

 fibres of the optic nerve are developed from some part other than the 

 optic stalk. Hence we are bound, I think, to conclude that the fibres 

 must be outgrowths from cells in either the brain or in the retina. 



Origin of the fibres of the Optic Nerve. 



I believe that with the exception of Froriep's (7) researches men- 

 tioned at the beginning of this paper, and Keibel's (13) short 

 statement, and of His's (12) expressions of opinion, there is no actual 

 evidence published of the growth of the fibres along the optic stalk, 

 either towards the brain or towards the eye. 



His's (12) evidence, though perhaps not complete, is very strong, 

 for he finds in human embryos of five weeks (13 mm.) the first trace 

 of nerve-fibres in th e retina as follows : 



" Ich selber finde die ersten Nervenfasern der Retina bei mensch- 

 lichen Embryonen von etwa 5 Wochen (13 mm.). Zu der Zeit zeigt 

 die Retina an ihrer Inuerseite ein von den Mullerschen Fasern 

 durchzogenes weites Raumsystem. Die Fasern bilden eine erst ditnne, 

 scharf auslaufende Schicht, und sie treten unter spitzen Winkeln aus 

 der austossenden Zellenlage hervor. Hier liegen kleine retortformig 

 gestaltete Neuroblasten, die mit ihren umgebogenen Spitzen in die 

 Fasern sich fortsetzen. Die zuerst gebildeten Opticus fasern enstam- 

 men somit den Zellen der Retina und wachsen central warts. In die 

 inneren Kornschicht dagegen finden sich zahlreiche Neuroblasten 

 welche ihre Spitzen nach auswiirts behren." 



All three authors, however, agree that the growth of the nerve- 

 fibres is from the eye to the brain. 



In support of their views I may add the result of my own 

 observations on the frog. 



Before explaining my figures on this point I must mention the 

 beautiful work of Cajal (3). 



The author, who made use of the Golgi and Weigert-Pal method, 

 describes the result of his observations on the optic lobes of adult 

 birds and of advanced embryos, amongst which were chicks of the 

 tenth day of incubation and upwards. 



