No. 2.] 



HISTOGENESIS OF THE RETINA. 



419 



form the optic cups they consist of but a single layer of 

 elongated cells with their nuclei located in their peripheral 

 ends. A continued dispersion and migration of the pigment 

 has also taken place, and is now more abundant in the stalk 

 than in the portion which will later form the optic cup." ^ 

 Fig. 3 gives the outline of the cells of the optic cup when it is 

 once well formed. The evagination is complete and at this 

 stage the mitoses are not in the cells of the cup, but almost 

 wholly at the point where the cup joins the brain. The pigment 

 outlines the cells very well and in general is well diffused 

 throughout the retina. The cell itself is filled with yolk 



Pig- 4- — Section through the brain and eye of Amblystoma (4 mm. long) seven- 

 teen days before hatching. Hardened in Flemming's solution. X 100 times. S, 

 spongioblast ; G, germinating cell. 



granules and its nucleus is located in its extreme peripheral end. 

 In an embryo which is somewhat older the optic cup is flat- 

 tened and the optic stalk has become smaller than it is in the 

 stage just described. This stage. Fig. 4, shows most beauti- 

 fully the formation of the first spongioblasts and is, therefore, 

 of the greatest value to us in our study. His ^ has already 



1 Eycleshymer, /. c, p. 193. 



2 His, Abhandl. d. konigl. sach. Gesellsch. d. Wissenschaften, Bd. XXVI, and 

 His u. Braune's Archiv, 1889. Also, His u. Braune's Archiv, Supplement-Rand, 

 1890, and Verhandl. d. X. Internationalen Med. Congresses, Bd. H, Berlin, 

 1891. 



