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 ihejvupil. 

 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 O f the 

 retinal layer is not a simple in- 

 vagiiiation, 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 

 the pupil appearing as a 



f 



FIG. 49. Plastic figure of hemi- 

 sected optic vesicle, lens and optic 



stalk of the frog. /, Choroid fis- dilatation of its upper end in the 



sure; /, lens; pc, posterior chamber -in e 



of eye; pi, outer or pigmented middle OI the CUp (FlgS. 49, 128). 



stalk; v, original cavity of optic pupil is referred to as the fundus 



vesicle. 



region. 



While the vesicle is invaginating the rudiment of the Jens 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 



