. 



OPTIC CUP AND LENS 



187 



ith its surface attachment dorsal and caudal, and its central end or stalk ventral 

 and cranial. The surface ectoderm now becomes thickened and pitted-in so as to 

 forma cup-shaped depression (fig. 180), which subsequently becomes converted into 

 a vesicle by the closure of its mouth. This is the rudiment of the lens, and pari 

 IKLSSU with its formation the optic vesicle becomes doubled up to form the optic 

 cap. The cavity of the cup is occupied at first by the lens vesicle, but later it 

 becomes opened out to form the cavity of the eyeball or vitreous chamber, while 



aud 



FIG. 180. TRANSVERSE SECTION HEAD OF A RABBIT-EMBKYO OF THE ELEVENTH DAY. (T. H. Bryce.) 



fb, fore-brain ; hb, hind-brain ; op.ves., optic vesicles ; lens, lens-plaque ; 

 aud., auditory vesicles. 



the original cavity of the optic vesicle is almost entirely obliterated, appearing 

 merely as a cleft between the outer and inner walls of the cup. 



Development of the lens. The rudiment of the lens is a disc-shaped ecto- 

 dermal plaque situated on the side of the head opposite the upper and outer aspect 

 of the optic vesicle (fig. 178). The plaque is at first closely applied to the outer 

 wall of the vesicle, but when this begins to be invaginated they draw apart some- 

 what, remaining connected, however, by protoplasmic strands (fig. 183). This 

 syncytial connexion is in all probability maintained during the formation and 



