316 OUTLINES OF CHORDATE DEVELOPMENT 



growing out from the ciliary ganglion. The neuroblasts form- 

 ing the ciliary ganglion appear to migrate from the neural 

 tube and from the profundus ganglion of the V nerve. These 

 afferent fibers seem to arise in the muscles of the eye-ball. 



The so-called / (Olfactory] and II (Optic) cranial nerves will 

 be considered in connection with the development of the sense 

 organs with which they are associated. 



II. THE SPECIAL SENSE ORGANS 



1. The Eye 



We have described in the preceding chapter the formation 

 of the optic vesicles from the primary fore-brain, and the 

 differentiation of the optic stalks, which remain related with 

 the ventral side of the diencephalon either side of and posterior 

 to the recessus opticus (Fig. 125). The optic stalks and vesicles 

 are the rudiments of only the essential, or sensory (recipient) 

 and nervous elements, of the eye. All of the other accessory 

 parts of this complex organ are differentiated from other tissues. 



While the tubular optic stalk remains comparative!} 7 short, 

 the optic vesicle enlarges rapidly and soon (thirty hours) 

 reaches the surface ectoderm of the head. The continued dila- 

 tion of the vesicle occurs mostly above the level of the stalk, 

 which therefore remains related with the ventral side of the 

 vesicle, as of the brain (Fig. 128). Now there appears a thick- 

 ening of the ectoderm opposite the optic vesicle; this is the 

 rudiment of the lens. Both the optic vesicle and the rudiment 

 of the lens then proceed to invaginate, independently, but in 

 the same direction (Figs. 127, 128). 



The invagination of the optic vesicle, which converts it into 

 the two-layered optic cup, is not a simple hemispherical invagi- 

 nation. The invaginating region begins about in the middle 

 of the outer wall of the vesicle and extends thence downward, 

 to the attachment of the optic stalk, as a vertical groove. The 

 distal wall of the vesicle rapidly folds in toward the proximal 

 wall and nearly obliterates the original cavity of the vesicle, 

 just as the invaginating endoderm of the Amphioxus gastrula 



