146 OUTLINES OF CHORDATE DEVELOPMENT 



immediately by the outgrowth of its free margin, which 

 gradually draws partially together, leaving a small opening 

 toward the surface of the head; this is the rudiment of the pupil. 

 In the optic cup then we distinguish an inner and an outer layer, 

 and a central cavity; these are respectively, the rudiments of 

 the true retinal layer, the pigment layer, and the posterior 

 chamber of the eye. 



During the growth and invagination of the optic cup the op- 

 tic stalk remains attached to its ventral side, so that the retinal 



layer of the cup is continuous with 

 the lower side of the optic stalk 

 (Fig. 49). The infolding of the 

 retinal layer is not a simple in- 

 vagination, but, owing to this ven- 

 tral attachment of the optic stalk, 

 the infold is continued as a groove 

 from the middle of the vesicle to 

 the ventral border of the cup 

 where it joins the stalk. This 

 groove remains narrowly open for 

 a time and is known as the choroid 

 fissure, the pupil appearing as a 

 dilatation of its upper end in the 

 middle of the cup (Figs. 49, 128). 

 That part of the cup opposite the 

 pupil is referred to as the fundus 

 region. 



While the vesicle is invaginating the rudiment of the lens is 

 formed as a thickening of the ectoderm opposite the pupillary 

 region (Figs. 48, 50). This thickening involves only the deeper 

 or nervous layer of the ectoderm and is, in all essential respects, 

 similar to the ganglionic portion of the ectodermal placodes 

 described in connection with the cranial nerves. This lens 

 placode is immediately anterior to the placode of the V cranial 

 nerve. By the time of hatching this forms a prominent rounded 

 thickening, which is cut off from the ectoderm as a solid 

 spheroidal cell mass (Fig. 50). After hollowing out internally 



FIG. 49. Plastic figure of hemi- 

 sected optic vesicle, lens and optic 

 stalk of the frog. /, Choroid fis- 

 sure; I, lens; pc, posterior chamber 

 of eye; pi, outer or pigmented 

 layer of optic cup; rl, inner or reti- 

 nal layer of optic cup; s, optic 

 stalk; v, original cavity of optic 

 vesicle. 



