DEVELOPMENT OF THE OPTIC NERVE OP VEETEBllATES. 93 



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 

 mentioned at the beginning of this paper, and KeibePs (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 the retina as follows : 



" Ich selber finde die ersten Nervenfasern der Retina bei 

 menschlichen Embryonen von etwa 5 Wochen (13 mm.). 

 Zu der Zeit zeigt die Retina an ihrer Innerseite ein von den 

 Mullerschen Fasern durchzogenes weites Raumsystem. Die 

 Fasern bilden eine erst diinne, scharf auslaufende Schicht, 

 und sie treten unter spitzen Winkeln aus der austossenden 

 Zellenlage hervor. Hier liegen kleine retortformig gestal- 

 tete Neuroblasten, die mit ihren umgebogenen Spitzen in 

 die Fasern sich fortsetzen. Die zuerst gebildeten Opticus 

 fasern enstammen somit den Zellen der Retina und wachsen 

 centralwarts. In die inneren Kornschicht dagegen finden 

 sich zahlreiche Neuroblasten welche ihre Spitzen nach aus- 

 warts 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. 



