No. 477] HISTOGENESIS OF RKTIXA 



617 



the undifferentiated margins. In this way everv section contains 

 in its various parts all the precedinu" stages ihrouuh which it ha> 

 passed. For instance a segment near tlie margin of a ten da\-" 



a segment tlirotigh the center of the retinal cnp from a vounger eve. 



margin, which invariably represent an earlier stage. 



In the growth of the retina as a wliole there are three well 

 defined periods: (1) the period of cell-mniti})iication. i2i the 

 period of readjustment, (3) the period of final (hfferentiation. 



the whole retina, which at the end of this stage ha> attained its 

 maximum width of l!).! -.. During the .succee<ling period of read- 

 ju.stment, from the eighth to tiie tenth (hiy. th.-re i> ^tiii a rajmi 



ber of nuclei i> henceforth practically Hxed. and the principal 



dental to this there is a sudden decrea.se in width to l.">0.". undoubt- 

 edly diu' t.. a stretching out of the surfac-e area. The >tage of 

 dilfereutiation. extending from thi. time to the end <.f incubation, 

 is nu.rked l.v the growth of ..vtoplasnnc processes and bv a grad- 



