320 OUTLINES OF CHORDATE DEVELOPMENT 



zone. Some of the retinal layer cells form neuroblasts, and 

 from these nerve processes (axons) grow out. along the inner 

 surface of the retina toward the attachment of the optic stalk. 

 Finally they extend, by way of the margin of the choroid 

 fissure, into the stalk, from all parts of the retina, and thence 

 into the wall of the diencephalon (optic thalami) to the optic 

 lobes. The consequent thickening of the ventral walls of the 

 optic stalks forming the optic tracts, leads to the disappearance 

 of the original cavity of the stalk. The solid stalks or optic 

 tracts, are commonly known as the II cranial or optic nerves 

 (Fig. 128). 



In the meantime the free inner margins of the choroid 

 fissure have extended up into the cavity of the optic cup 

 (posterior chamber), and finally they fuse forming a low ridge, 

 the whole length of the original fissure, enclosing between 

 them a bit of mesenchyme which had extended into the fissure 

 from without. Finally the lips of the fissure fuse together 

 externally also, and the internal ridge is recognized as the 

 rudiment of the pecten of the eye. This enlarges rapidly and 

 ultimately forms a folded, fan-like organ, projecting some 

 distance upward into the posterior chamber. 



The general content of the posterior chamber, known as the 

 vitreous humor, is formed from the innermost cells of the retinal 

 and lenticular zones. These cells send out branching processes 

 through the chamber and later add a secretion, the two materials 

 together making up the humor. 



Just after the rudiment of the lens separates from the 

 surface ectoderm, its inner wall becomes thickened by the 

 elongation of its single layer of cells. Finally then- elongation 

 becomes so marked that the original cavity of the lens vesicle 

 is obliterated and the lens become solid. The epithelium on 

 its outer face remains simple. This primary lens vesicle forms 

 only the nucleus of the final structure, for around this are 

 laid down, beginning about the eighth day, a large number of 

 concentric layers of cells, derived from the periphery of the 

 lens region, where the thin outer lens epithelium becomes con- 

 tinuous with the thick inner layer. These cells are formed 



