16 WILLIAM H. F. ADDISON AND HAROLD W. HOW 



the tarsal glands are distinctly open. The hairs and hair fol- 

 licles of the marginal region of the lids are large and numerous. 

 The whole ajipearance suggests the completed eyelids. 



Fourteen-day rat 



In some animals the final separation takes place on this day; 

 in others, on the fifteenth day, and in a relatively small number 

 the process is delayed until the sixteenth or seventeenth day. 

 As mentioned above, the first o])ening usually a]:)pears in the 

 middle region of the lids and proceeds in both directions, but 

 usually reaches the inner canthus first. For some hours after 

 the margins have separated at the inner canthus the lateral fourth 

 of the lids may still be fused. The time necessary for complete 

 opening after the first slit appears is often twelve hours. The 

 lids on the tw^o sides of the head of an animal do not necessarih' 

 separate at the same time. One may be entirely open before 

 the first slit appears in the other. According to the observations 

 of Dr. H. D. King, it often happens that in the same litter the 

 eyelids of the females open before the eyelids of the males (Don- 

 aldson, '15). This circumstance is in line with the general pre- 

 cocity of the female at this period, as compared with the male. 

 It sometimes happens that the eyelids are only partially sepa- 

 rated for days after the usual time, and this gives them the 

 appearance of being 'small-eyed' animals. In such cases, 

 according to Dr. J. M. Stotsenburg, of The Wistar Institute, the 

 separation may be easily completed by gentle pressure upon the 

 lids with the tips of one's fingers. 



Structure of retina at time of fusion and of opening of lids 



At the time of fusion of the lids in the rat, on the eighteenth 

 fetal day, the retina shows an early stage of development. The 

 cells derived from the inner layer of the optic cup still form one 

 continuous layer, but arranged in two zones (fig. B). This is 

 also the condition present in the human retina at the time of 

 fusion of the lids (Bach and Seef elder, '14). Of the two strata, 

 the outer one is composed of closely arranged deep-staining cells, 



