PETROMYZON 



23 



conjunctiva. The eye may then be regarded as a closed hollow 

 ball, with the enclosed chamber divided into two by the lens : 



Fig. 14. — Diagram showing the method of origin of the eyes in chordate 

 animals and the relations of the sensory cells. 



A, before the formation of the nerve-cord the sensory cells (sc) are on 

 the outer surface ; B, when the nerve-cord has been formed the sensory 

 cells line its cavity (c) ; C, formation of the optic vesicles (ov) ; D, origin 

 of the lens (/) from the epidermis, conversion of the optic vesicles into 

 optic cups (oc) and formation of the pineal vesicle (pv) ; E, condition with 

 completely formed eyes, ac, anterior chamber ; ch, choroid ; cm, ciliary 

 muscle ; en, conjunctiva ; cr, cornea ; e, epidermis ; i, iris ; /, lens ; on, 

 optic nerve ; p, pigment layer ; pc, posterior chamber ; pi, pineal lens ; 

 pr, pineal retina (erect) ; r, retina (inverted) ; s, sclerotic. 



viz., a posterior chamber between the retina and the lens, and 

 an anterior chamber between the lens and the cornea. The 

 posterior chamber contains a jelly-like substance called the 



