THE NERVOUS SYSTEM 423 



becomes the tapetum nigrum, a layer which, together with the 

 blood capillaries later to be associated with it, will become the 

 chorioid coat. The stalk, although not directly transformed 

 into the optic nerve, forms the path along which it develops 

 and thus marks its final position. (For the details of this 

 cf. the last part of Chapter XL) 



During the time at which the optic cup has been forming by 

 a turning in of the outer part of the vesicle, an associated 

 process takes place in the ectoderm directly opposite the cup. 

 This process consists of an inpushing from without on the part 

 of this ectoderm, the inpushing going rapidly through the 

 stages of a simple depression, a depression with a narrowed 

 neck, and finally that of a spherical vesicle entirely cut off from 

 its layer of origin. That this may once have been the essential 

 sense organ to supply the needs of which the diverticulum from 

 the brain may have originated, seems likely from the similarity 

 of its early development to that of certain actual sense organs, 

 especially the otic capsule, which develops into the inner ear. 

 This latter, as will be shown later, appears to have been at 

 first merely a single unit of the system known as the " lateral 

 line organs," and the lens, although no longer sensory in func- 

 tion, may with some probability be referred to the same source. 

 In all present-day vertebrates, however, it is no longer sensory, 

 but develops into an auxiliary though essential organ of the 

 eye, the crystalline lens. This is accomplished by an enormous 

 thickening of the inner wall of the vesicle, which finally fills up 

 the entire lumen, leaving the outer wall to fit over it in the 

 form of a protecting epithelium. During later development 

 the eye receives its vitreous humor, its blood-vessels, its sclero- 

 tic coat and other essential parts from the surrounding tissue, 

 mainly the mesenchyme, and develops into the adult form. 



But one other diverticulum arises from the diencephalon, 

 and that one is directed downwards from the middle of its 

 floor. Like the lateral eyes, it does not form a complete organ 

 in itself, but unites with a similar diverticulum which develops 

 upward from the roof of the mouth, and together they form an 

 organ of 'slight functionaHnrgiortance^in respect to which the 



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