DEVELOPMENT OF EYE OF SPARROW 277 



THE LENS 



Though the development of the lens of the sparrow follows in 

 general that of other vertebrates, some modifications are found 

 which warrant a brief description and comparison of the dif- 

 ferent stages. 



Figure 6 shows that the lens is developed from the extoderm. 

 This layer at first thickens (A) immediately over the area of 

 contact with the optic vesicle. Rabl ('99) states that the first 

 thickening of the ectoderm in the formation of the lens in the 

 chick occurs at the age of twenty hours' incubation. Other 

 authors say it varies between eighteen and twenty-one hours or 

 more. Froriep ('06) concludes that it occurs in the chick at the 

 end of the second or the beginning of the third day. A very 

 marked thickening is seen in the forty-hour chick (fig. 43, L). 



Keibel and Elze ('08) show a recognizable thickening epithe- 

 lial plate, the beginning of the lens, in a 4-mm. mammalian 

 embryo. This corresponds to the twenty-hour chick embryo. 



This thickened area soon becomes concave exteriorly and 

 sinks in to form a pit. This occurs in the chick (Duval, '89) 

 when 5 to 7 mm. long, with 22 to 25 somites at the age of forty- 

 six to fifty-two hours' incubation. Text-figure 6, B, and 

 figure 44, Lc, show this invagination of the epithelium in the 

 chick at the age of fifty to fifty-six hours' incubation. This is 

 at a slightly later age and shows that the edges of the depression 

 are beginning to approach each other to form the lens vesicle. 

 With this invagination, the wall of the optic vesicle in contact 

 with this area is apparently pushed inward, gradually oblit- 

 erating its cavity, and finally forming the double-walled optic cup. 



It has been experimentally demonstrated by Spemann ('01) 

 and Lewis ('04) that the optic cup is not formed by pressure 

 exerted by the developing lens. They have shown that when the 

 optical vesicles are implanted in abnormal situations the optic 

 cups will be formed without the development of a lens. They 

 also conclude that the formation of the lens is due to the stimulus 

 caused by the optic vesicle's coming in contact with the ectoderm. 

 This stimulus will cause any portion of the ectoderm brought in 

 contact with the optic vesicle to form a lens. The formation of 



