DEVELOPMENT OF EYELIDS OF ALBINO RAT 



17 



and the inner one has its cells more palely staining and not 

 so compactly arranged. No definitive visual cells are differen- 

 tiated and no rods have yet developed. 



During the period of the rat's development, when the lids are 

 joined together, the retina undergoes its histogenesis. By the 



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B. 



Pigs. B and C Two drawings to show comparison of states of development of 

 retina at time of formation of lids and at time of their opening. In both the inter- 

 nal margin of the retina is shown toward the top of the page. X 250. 



Fig. B Retina of eighteen-day albino rat fetus at time of formation and fusion 

 of lids, showing a very early stage of development and consisting principally of 

 two broad zones of epithelial cells. 



Fig. C Retina of fourteen-day albino rat at time of disjunction of lids, show- 

 ing all the definitive layers of the functional retina. The proportions of the var- 

 ious layers, however, change with the further growth of the entire eyeball. 



time of opening of the lid all the layers of the mature retina are 

 clearly delimited (fig. C), though subsequent growth changes 

 alter their relative proportions somewhat. 



Thus it is seen that there is a close correlation between the 

 time of opening of the lids and the arrival of the retina at a 

 functionally active condition. Though the mechanism by which 



