ORGANS OF SPECIAL SENSE 275 



ment layer of the retina. About the middle of the fourth day 

 pigment begins to develop in the outer layer and extends through- 

 out it, even to the distal portion of the optic-stalk at first (Ucke, 

 '91). The histogenesis of the retina of the chick has been de- 

 scribed by Weysse (1906). 



2. The Vitreous Humor (Corpus Vitreum). Until compara- 

 tively recently embryologists have adhered to the view stated 

 by Schoeler (1848) and Kolliker (1861) that the vitreous body 

 arises from mesenchymal cells that enter the eyeball through 

 the choroid fissure. The fact that the embryonic vitreous humor 

 of birds is almost entirely devoid of cells was a serious difficulty. 

 The cells are in fact so scanty as to be absent in many entire 

 sections. Moreover, in character thev resemble embrvonic 

 blood-cells and not mesenchyme, and disappear entirely by the 

 eighth day. It seems impossible that they should play any 

 important part in the origin of the massive vitreous body. Re- 

 searches of the last few years have demonstrated that the vitreous 

 body is primarily of ectodermal origin, its fibers arising as processes 

 of cells of the inner layer of the optic cup and the matrix as 

 secretion. According to some the cells of the lens are responsible 

 wholly (Lenhossek) or in part (S/ili) for the fibers; this view, 

 however, has been strongly combatted (Kolliker and Rabl) and 

 requires further evidence to substantiate it. 



Both retinal and csecal parts of the cup take part in the forma- 

 tion of the fibers of the vitreous body; the retinal part is at first 

 the most important, and the primary vitreous body is almost 

 entirely retinal iu its origin. But after the csecal part is differ- 

 entiated the activity of the retinal part becomes less, and the 

 greater part of the fibers of the vitreous body appears to be 

 formed from cells of the csecal part, that send out branching 

 and anastomosing processes into the posterior chamber. There 

 is no sharp boundary between the fibers that form the vitreous 

 body and those that form the zonula; and the fibers of the latter 

 may be regarded as homologous to those of the former. The 

 matrix of the embryonic vitreous body may be regarded as a 

 secretion of the walls of the optic cup. Later, the secretion 

 appears to be confined to the ciliary processes. It is possible 

 that the mesenchyme plays some part in the formation of the 

 vitreous body after the formation of the pecten begins; but there 

 is no evidence that it does so at first. 



