DEVELOPMENT OF EYE OF SPARROW 287 



and that, later, the mesodermic portion is formed. They are 

 not clear as to just what takes place at this time. 



In mammals there is a migration of mesodermal tissue into 

 the optic cup, accompanied by blood-vessels which doubtless 

 play an important part in the formation of the vitreous body. 

 This was noticed in the &-mm. pig embryo at a time when the 

 lens vesicle had just been formed. The artery lentis, which is a 

 branch of the arteria centrahs retinae, finally branches profusely 

 over the posterior surface of the lens and forms the capsula 

 vasculosa lentis. These vessels appear in the human embryo 

 about the second month of fetal life and disappear about the 

 seventh month. 



In the bird I have found no migration of mesodermal cells 

 nor of blood-vessels as described in mammals. I have seen no 

 evidence of a capsula vasculosa lentis. All that I have been able 

 to see in my sections is the progressive formation of the vitreous 

 chamber which is filled with a clear colorless substance. The 

 bird does not possess an artery centralis retinae like that of 

 mammals. Neither does it have an arteria lentis (Froriep). 

 The internal structures of the bird eye are apparently nourished 

 from the blood supply to the pecten. Since the pecten is devel- 

 oped rather late in the formation of the eye, it would appear 

 that the vitreous body is developed as a clear secretion from the 

 retinal layer of the optic cup, or from the mesodermic cells 

 adjacent to the choroid fissure and rim of the cup, or possibly 

 from both these sources. 



The first appearance of the vitreous chamber occurs in the 

 bird (chick) soon after the beginning of the invagination of the 

 optic vesicle to form the optic cup at about the time of the com- 

 plete closure of the lens vesicle. This is sfeen in the sixty-four- 

 hour chick embryo (fig. 45). The substance filUng this space 

 appears clear and structureless. It contains no visible meso- 

 derm cells or blood-vessels. The only change noted in further 

 development is a relative increase in size as the w^hole eye grows 

 larger. Later, the very vascular pecten is developed, but noth- 

 ing else of a structural character have I been able to make out 

 in the vitreous chamber during the complete formation of the 

 eye. 



