DEVELOPMENT OF EYE OF SPARROW 273 



(Ac), which is still limited to this region, is now completely 

 separated from the posterior cavities. The retina and pigment 

 layers are relatively very much thinner and have begun to bend 

 around so as to present a flat face to the surface of the lens. 

 They are also beginning to show evidences of folding. This is 

 an early stage in the development of the ciliary processes. 



According to Kessler ('77), the first appearance of the ciliary 

 bodies is seen in the chick on the ninth or tenth day of incubation. 

 This is almost one-haK the incubation period of the chick. 

 According to my observations, the first appearance of the forma- 

 tion of the ciliary bodies in the sparrow occurs at a relatively 

 later age of incubation. In this folding the retinal, the pigment, 

 and the mesenchymatic portions are all involved. In man the 

 ciliary bodies first appear, according to Kolliker ('83), at the end 

 of the second or the beginning of the third month. Keibel and 

 Mall ('12) state that the ciliary bodies are recognizable in the 

 human fetus at the end of the fifth month. 



The distal portion of the pigment layer in the sparrow is now 

 well pigmented and touches the posterior surface of the develop- 

 ing iris, although the line of separation can still be easily seen. 

 The mesenchymatous cells are differentiating, so that the scleral 

 and chorioid portions are easily distinguished. Both of these 

 layers become rapidly thinner as they are followed posteriorly 

 from the lens. The iris now shows a clear connection with the 

 developing chorioid and the cornea with the sclera. 



Figure 39 shows the development at ten days. The cornea has 

 increased in thickness by increase in the substantia propria. 

 Its total thickness now measures 0.100 mm. The thickness of 

 the different parts is: membrane of Descemet, 0.006; substantia 

 propria, 0.074; epithelium, 0.020 mm. The iris extends farther 

 over the front of the lens, but mainly as a continuation of the 

 thin layer of mesenchjine. The pigmented portion of the retina 

 is much thinner at its free margin, but posteriorly it has thickened 

 The retinal layer is also thinner at the free margin. The aqueous 

 chamber is still limited to this angle. 



By the twelfth day of incubation (almost the time of hatching) 

 marked changes are noticed in the development of the iris and 



