No. I.] 



OPTIC VESICLES IN AMPHIBIA. 



193 



following quotation shows: "In der Mittelebene zeigt sich 

 gar kein Merkzeichen, welches auf Duplicitat der Anlage 

 deutete, dieselbe ist vielmehr zunachst eine unpaarige. . . . 

 Die spater paarige Erscheinung des Organs wird nur darin 

 angedeutet, dass in den lateralen Regionen der Erweiterung 

 beiderseits Mitosen auftreten, die am Boden fehlen." 



During the later stages of development the walls of the 

 vesicles become thinner so that just before they invaginate 



Fig. V. 

 Transverse section through optic vesicles. 



to 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, 



A more extended and detailed account, together with a dis- 

 cussion of the theoretical bearing of these facts, is reserved 

 for a later paper. 



Biological Laboratory, 



University of Chicago. 



