4iS MALL. [Vol. VIII. 



With these ideas clearly in mind it is quite easy to understand 

 why the optic nerve perforates the retina in the vertebrate eye 

 and passes directly to the brain in the invertebrate. They also 

 tell us why primitive eyes are composed of but a single layer of 

 cells sending nerve fibers directly to the brain, and why in the 

 more complex eyes a chain of cells is used for the same purpose. 



In the present study it has been my aim to follow the for- 

 mation of the retina from the time the optic vesicle is well 

 marked to the beginning of the optic nerve. Eycleshymer ^ has 

 already shown us that in Amblystoma and Necturus the optic 

 cups are well marked long before the neural canal begins to 



Fig. 3. — Section through the eye of Amblystoma (3.5 mm. long). X 66 times. 

 P'rom a drawing by Mr. Eycleshymer. O. V., optic vesicle. 



close by pigmented areas which are slightly cup-shaped, and are 

 made up of a single layer of cylindrical cells with the pigment 

 at either end of them. As the areas grow more and more to 

 form cups the mitoses are more numerous in them than in the 

 surrounding ectoderm which is to form the neural canal. The 

 pigment gradually becomes more and more diffuse and serves 

 to a great extent in following the differentiation of the retina. 

 "During the later stages of development the walls of the 

 vesicles become thinner, so that just before they invaginate to 



1 Eycleshymer, Journal of Morphology, Vol. VIII. 



