144 OUTLINES OF CHORDATE DEVELOPMENT 



forming additional sympathetic elements. From the sympa- 

 thetic cords, cells also migrate peripherally, forming the per- 

 ipheral sympathetic ganglia in connection with the great blood 

 vessels and the thoracic and abdominal viscera. The ganglion 

 of the III cranial nerve is also sympathetic in character, but 

 its origin is uncertain in the frog, as is also the origin of the sym- 

 pathetic fibers of the head region in general. 



II. THE SPECIAL SENSE ORGANS 



1. The Eye 



In the preceding chapter we described the earliest traces of 

 the eyes. These consist of a pair of patches in the superficial 

 ectoderm of the medullary plate, before this has begun to fold 



med- 



FIG. 47. Transverse section through the anterior part of the medullary plate 

 of an embryo of R. palustris, in which the medullary ridges are just forming. 

 From Froriep (Hertwig's Handbuch, etc.), after Eycleshymer. au, Optic grooves; 

 en, endoderm; ep, ectoderm (epidermis); med, medullary plate; ms, mesoderm. 



together. The cells of these patches are distinguished by their 

 comparatively large size and by the presence of pigment in 

 their outer ends (Fig. 47). When the medullary plate folds 

 into a tube these patches are carried inward and are left toward 

 the antero-ventral border of the fore-brain (telencephalon). 

 Their __pjgment then gradually disappears, but it is clear that 

 their originally free ends now border the cavity of the brain. 

 Before the brain folds have entirely closed together, the regions 

 surrounding and including these patches have already evag- 

 inated and formed the pair of optic stalks and vesicles extending 



