13 



This simple solution of the complex vitreous body problem, how- 

 ever, meets with some difficulties in the subject under consideration 

 here. The above-mentioned investigators arrived at their conclusion 

 by the study of human material almost exclusively. But in the eye 

 of the pig, the neuroglia mantle has been observed around only 

 about one-third of the length of the main trunk of the hyaloid ar- 

 tery ; it has never been shown to surround any of its branches. Our 

 own observations agree fully with those of Wolfrum. A number 

 of sections show the neuroglia mantle surrounding a portion of the 

 hyaloid artery proper ; but the branches exhibit only the usual vas- 

 cular endothelium, which, as further investigation will show, enters 

 into a close relationship with the vitreous body. This complex 

 relationship between the vitreous body and the mesodermal ele- 

 ments of the blood vessels prevents our accepting, for the eye of 

 the pig at least, the simple and attractive theory of the neuroglia 

 origin and nature of the vitreous body. 



But there are other elements found in the optic cup at this time 

 of development which claim our attention. Together with the hyaloid 

 artery, some free mesoderm cells likewise enter the optic cup 

 and almost simultaneously we witness the invasion of the 

 vitreous body by a large mass of mesoderm through the perilenticu- 

 lar opening (Figure 2). The function of these mesodermal ele- 

 ments has never been satisfactorily accounted for. Several cir- 

 cumstances, to which, as far as we know, attention has not yet been 

 called, incline us to the belief that these mesoderm cells are pri- 

 marily vaso-formative, aiding in the building up of the complex 

 vascular system of the embryonic mammalian eye : 1. the invasion 

 of the vitreous body by mesoderm is restricted to a comparatively 

 short period of time, which corresponds to the period of the most 

 rapid growth and expansion of the vascular system, and ceases as 

 soon as the latter has attained its highest development (Figures 2 

 and 12) ; 2. while at first the mesodermal elements are found scat- 

 tered throughout the larger part of the vitreous body, most of them 

 soon congregate in the vicinity of the blood vessels, the rest of the 

 vitreous body being at that time almost devoid of mesoderm ; 3. that 

 there exists some relationship between the vascular system of the 

 vitreous body and the mesodermal elements, seems to be indicated 

 also by the fact that in the eye of the birds, which even during em- 

 bryonic development never contains blood vessels, the number of 

 mesoderm cells is exceedingly small. Many authors even doubt the 



