618 



THE AMERICAN NATURALIST [Vol. XL 



of the retinal cup and thence radiating outwards on every side 

 toward the margins, until at the end of this period the layer comes 

 to be composed of a single phalanx of nuclei (Fig. 7) . This re- 

 adjustment takes place in the direction of the internal limiting 

 membrane as is shown by the fact that there is no corresponding 

 increase in the width of the nerve-fiber layer. Thus in its develop- 

 ment the ganglion-cell layer actually decreases in width from 28.6 

 to 10.4/*. 



Part of \hr imclci of this layer are the same nuclei that were 

 seen in the early retina imme- 



. - _e.l.m. (liately after the formation of the 



(:) . r > -r/ "- optic Vtip (Fig. 1), and the re- 



' '' h.niH'.l 1,\ th," division of the 



-vrinit.al nuclei. All are of the 



t"' ' • ' ' i '""^ ni ^ame size as those foiuid in the 



einhrv', All th,- nn'i.i n! ml,, -i.'imi Undifferentiated condition; 



i'a,- '''!'!'''r|''',.-"M.h' ^'^^^ s^iy- after division each 

 n - • I P ^;^)\v^ to tlic sizc of the parent 



i' ' ~, , nucleus, a fact also true of the 



aina. rincs or horizontal nuclei. At 

 the a.ivcul of tiic tliinl Magcof gn)\sth, u ith its atteiidant increase 

 in the amount of cytoplasm, these cells become nuiIti[>olar and 

 gradually approach nearer and nearer the condition of the adult 



appcaran<.e three fourths of a <lav later. irillHl'Tw. Uvo.nf 1 

 complete law-r l>y h.elf until the he^inniug of the period of rea.l- 

 justinent when the outer reticular layer aj)pears and separates it 

 from the future external nuclear layer; but long before this, 

 differentiation has already taken place. 



