12 



vitreous body (Figure 4). It soon gives rise to a number of branches 

 which fill out the larger part of the space between the retina and the 

 lens (Figure 2). Their tendency, however, is always towards the 

 lens, which finally is completely surrounded by blood vessels furnish- 

 ing it with nourishment during the period of its most rapid growth. 

 Later there arises from the hyaloid artery near its entrance into the 

 optic cup a second group of branches which radiate in all directions 

 through the outer layer of the vitreous body near the retina. This 

 condition is observable especially in embryos ranging from 35 to 100 

 mm in length (Figure 14). The larger part of the vitreous body of 

 such embryos is again free from blood vessels. 



A closer examination of the structure of the blood vessels shows 

 that their walls are generally thin, consisting of a layer of endothelial 

 cells to which a connective tissue cell is attached here and there. This 

 structure is common to the main trunk as well as to all the branches 

 of the hyaloid system at this time. Later, however, the main trunk of 

 the hyaloid artery seems to be surrounded by an additional layer of 

 cells enveloping the vascular endothelium like a mantle. This mantle 

 layer has been classed by some authors among the neuroglia tissues 

 of the central nervous system. Its intimate connection with the sup- 

 porting tissues of the optic nerve, the structure and the epithelial ar- 

 rangement of its cells, and the reaction of the latter to stains in a 

 manner not unlike the neuroglia cells of the optic nerve, are said to 

 prove the identity of this cell hiyer with the supporting tissues of the 

 optic nerve (Mavas and Magitot, p. 129) . For the eye of the pig, this 

 structure has been described at length by Wolf rum (p. 249) , to whom 

 the interested reader is referred. 



While Seefelder at first observed the neuroglia mantle around 

 only the main trunk of the hyaloid artery, Mavas and Magitot found 

 it enveloping not only the main trunk, but all the branches of the 

 hyaloid system, thus interposing an ectodermal sheath of neuroglia 

 tissue between the mesodermal elements of the blood vessels and 

 the vitreous body and effectively preventing their union. Ac- 

 cording to these authors, therefore, the vitreous body is at this time 

 free from all mesodermal admixture, and represents a purely ecto- 

 dermal formation consisting of a framework of fibers, the product 

 partly of the supporting tissues of the retina and partly of the 

 neuroglia cells of the optic nerve. The vitreous body would then 

 have to be regarded as a purely neuroglia tissue derived ultimately 

 from the central nervous system (Mavas and Magitot, p. 132). 



