DEVELOPMENT OP THE OPTIC NERVE OP VERTEBRATES. 177 



seems to me that a consideration of the manner in which the Vertebrate 

 eye was evolved almost necessitates the occurrence of a gap at a 

 certain stage. 



Development of Optic Nerve in Chick. 



I have followed also the development of the optic nerve in the 

 chick, and find that the mode of development is essentially similar to 

 that described .above for the frog, Rana temporaria. 



In chicks of four days thick nerve-fibres may be found in the 

 retina, radiating towards and into the just beginning choroidal fissure, 

 but can be traced no further. In five-day chicks the fibres are 

 thinner, and can be easily traced into the choroidal fissure, but along 

 the optic stalk near to the brain there is no trace of nerve fibres. 



In six days fibres can be traced all the way to the brain. 



Summary. 



1. The optic stalk takes no part in the formation of the nervous 

 parts of the organ of sight. 



2. The optic stalk becomes broken down and the cells composing it 

 separated from one another, partly by the mechanical stretching due 

 to the growth of the optic nerve, partly by the growth in between the 

 several cells of the nerve-fibres. 



3. The optic nerve is developed independently of the optic stalk, 

 the nerve-fibres lying along the posterior border of the stalk, and at 

 first entirely outside it ; but on the breaking down of the stalk some 

 of the nerve-fibres grow in between the cells. 



4. The great majority of fibres forming the optic nerve arise as 

 outgrowths from nerve-cells in the retina, and grow towards and into 

 the brain. 



5. According to Cajal's researches certain fibres also exist which 

 would seem to grow from the central nervous system to the retina, but 

 these I have not been able to find. 



6. The nerve-fibres pass over the ventral edge of the optic cup, and 

 thereby cause the formation of the choroidal fissure. 



7. The choroidal fissure of the embyro represents a conditiou in 

 the evolution of the eye which was persistent in the adult prior to the 

 formation of a lens. 



M 



