DEVELOPMENT OF THE OPTIC NERVE OF VEETEBEATES. 87 



until the present time by nearly every English writer on the 

 subject. 



There are apparently still those who hesitate to accept the 

 more recent views as to the origin of the optic nerve. As 

 examples I will quote passages from the last editions of two 

 widely read educational works, namely, ' A Text-book of Phy- 

 siology, by Foster, and 'The Frog,' by Marshall. 



On p. 1141 of the former work Professor Foster writes: 

 " The cup becomes what we may speak of broadly as the 

 retina, and we may call it the optic or retinal cup ; the solid 

 stalk becomes the optic nerve." And again, on p. 1142, "At 

 the time when the epithelial cells of the stalk of the retinal 

 cup are developed into the fibres of the optic nerve these 

 become connected with the elements of the inner or retinal 

 wall of the cup ; they pierce the outer wall of pigment epi- 

 thelium, making no connections with the cells of that outer 

 wall." 



On p. 131 of the latter work Professor Marshall writes, 

 "The optic vesicles have already been described as arising at 

 a very early period as lateral outgrowths from the fore-brain ; 

 these soon become constricted at their necks, so as to be con- 

 nected with the brain by narrow stalks, which ultimately 

 become the optic nerves." 



In view of this difference of opinion, which still exists con- 

 cerning the origin of the fibres of the optic nerve, I think an 

 account of a research made some months ago upon this subject 

 in the frog and chick may be of some interest, especially as 

 certain structures connected with the development of the eye 

 and other parts of the nervous system may be more easily 

 understood by appreciation of the fact that the optic nerve 

 and optic stalk are two entirely separate structures. 



Relation of the Optic Stalk to the Optic Nerve. 



The two views held at present are — 1. The optic nerve is 

 formed by the differentiation of the cells of the optic stalk 

 into nerve-fibres, which subsequently lose connection with the 



