119 



Fig. 8. 



Fig. 9. 



--o\>y- 





Ab 





^^M 



m^'^ 







---ov- 



c*t..v. 





Fig. 8. Embryo at about the 29th hour of development showing an early stage of 

 the brain vesicles. 



Fig. 9. Embryo at about the 31st hour of incubation showing the succession of 

 cerebral vesicles, op. v optic vesicle, cb. v. cerebral vesicle. Ht heart, au auditory 

 saucer. 



the hinder portion also, and, may we not be permitted to suggest that 

 we have here a trace of an ancestral condition, in which the sensory 

 epithelium, from which the eyes have been differentiated, extended 

 backwards in segmental patches. 



The chick is not the only animal in which "accessory optic ve- 

 sicles" are known to exist. I have observed similar structures on 

 the cephalic plate, and in the neural groove of Acanthias, and have 

 shown photographs, figures and sections of the same^). 



1) The Optic Vesicles of Elasmobranchs, and their serial Relation 

 to other Structures on the Cephalic Plate. Journ. Morph., Vol. IX, 

 January 1894. And more especially, .Journ. Morph., Vol. IX, Dec. 1895, 

 p. 556—561. 



