14 



presence of mesoderm in the chick's eye. Lillie says of these cells 

 that "in character they resemble embryonic blood-cells and not 

 mesenchyme, and disappear entirely by the eighth day" (p. 275). 



That not all the mesoderm cells are, however, used in the forma- 

 tion of the blood vessels is evident from the fact that at all stages 

 of development a number of them are found isolated in the vitreous 

 body. It is not improbable, as some authors maintain, that these 

 free mesoderm cells by a process of disintegration and degeneration 

 contribute to the production of the fluid parts of the vitreous body. 

 Accordingly, we have at this period of development two distinct 

 mesodermal elements in the vitreous body, the mesoderm tissue of 

 the blood vessels and a number of free mesoderm cells (Figure 13). 



The question now arises as to what influence the hyaloid system 

 and the free mesoderm cells have on the further development of 

 the vitreous body. Do they modify the originally ectodermal nature 

 of this tissue in such wise that we are forced to admit a mixed struc- 

 ture consisting of various parts, derivatives of both the outer and 

 the middle germ layer ? 



It can not be denied that a close relationship arises between the 

 fibers of the vitreous body and the walls of the blood vessels. The 

 endothelium of the latter sends out a large number of fibers which 

 penetrate into the surrounding vitreous body. They are short and 

 delicate and never attain the massiveness of the vitreous fibers. 

 Their purpose may be solely to secure a hold for the blood vessels 

 in the loose surrounding tissue, a phenomenon not infrequently 

 observed in embryonic structures. The retinal fibers, on the other 

 hand, enter into direct union with the blood vessels. It is not diffi- 

 cult to trace many fibers in their entire course from the retina to the 

 endothelial lining of the blood vessels, forming a protoplasmic 

 bridge between the two. While the phenomenon can be observed 

 at almost any stage of development, it is especially prominent in 

 embryos ranging from 50 to 130 mm in length. Here at times we 

 see the fibers apparently radiating from the blood vessels like the 

 radii of a wheel (Figure 14). It is probable that in this way nutri- 

 ment is carried from the blood vessels to the vitreous body and also 

 to the retina, which for a long time is deprived of a vascular system 

 of its own. 



A. V. Szily, who has devoted much labor to the study of this 

 peculiar phenomenon of the concrescence of ectodermal vitreous 



