DEVELOPMEiNT OF THR OPTIO NERVE OF VERTEBRATES. 101 



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 essen- 

 tially 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 com- 

 posing 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 condi- 

 tion in the evolution of the eye which was persistent in the 

 adult prior to the formation of a lens. 



VOL. XXXIV, PART II. — NEW SEE. H 



