117 



be designated "accessory optic vesicles". It will be shown 

 presently, that they are neither cerebral vesicles nor the early stages 

 of those structures, and, therefore, the term suggested is more appro- 

 priate. 



The change of the optic vesicle from a very elongated condition; 

 as shown in Fig. 1, into a shorter more rounded one, as shown in 

 Figs. 2, 4, 5, 6 etc., is the usual course of events in the development 

 of the head of the chick. Every observer of this embryo has noticed, 

 that when the optic vesicles begin to appear, at about the 24th hour, 

 they are very much elongated. The front end of the neural tube is 



Fig. 1. 



Fig. 2. 



Fig. 3. 



M^ 





i? 





%t 



->-_>^.<,o. 





Mi '='V^- 









?/ ; 



a ■'riV- it 7 



x^h ^^ \ - . q acv. 





^ 





t> cvc \ . 







f- '■;--i^---^»•.bV,. 







Fif:. 1. Cliick embryo about 24 hours old, showiug optic ridge, op.R\ segmental 

 divisions, 1, 2, o etc.; mesoblastic somites, m.so; primitive streak, ^r. «(. 



Fig. 2, Chick embryo between the 24th and 25th hour of incubation showing the 

 optic vesicles, op.v, and six pairs of "accessory optic vesicles", ac. v. 1 to ac.v.6. 



Fig. 3. Section of the embryo represented in Fig. 2 , taken in the horizontal 

 plane and passing through the system of optic vesicles. 



