270 JAMES ROLLIN SLONAKER 



With three days of incubation this rather blunt projection has 

 extended some distance farther toward the front of the lens 

 (fig. 32). It appears as an undifferentiated mass of mesodermal 

 cells. The conjunctival epithelium is still closely applied to 

 the lens. 



By the fourth day (fig. 33) the growth from the mesoderm 

 has extended a considerable distance over the front of the lens, 

 and in doing this it has pushed the epithelial layer away from 

 the lens. The cells of this projection have become very much 

 elongated, somewhat spindle-shaped and for the greater part are 

 arranged in a single row. This thin projection fills only a part 

 of the space formed by the separation of the epithelium from 

 the lens. It Hes closer to the surface of the lens than to the 

 epithelium. No evidence of the developing iris is seen at this 

 age. 



Since this projecting mass of cells seems to be a distinct out- 

 growth from the mesoderm and at first is not even in contact 

 with the epidermis, one is forced to disagree with Kessler (77), 

 who claims it is a product of the secretion of the epidermis, and 

 to accept KoUiker's ('83) view that it is mesenchymatous in 

 origin. The development of the mammalian eye also substan- 

 tiates this view. 



At the fifth day (fig. 34) the membrane of Descemet is a single 

 layer of spindle-shaped cells extending over the entire surface 

 of the lens. ' A space separates the epithelium from this layer 

 of mesenchyme cells throughout its entire extent. Near the 

 periphery this layer is somewhat thickened (due to the migration 

 of mesenchyme cells from the periphery into it) and is several 

 cells in width. This condition corresponds very closely to the 

 fifteen-day chick as described by Kessler and to the fourth month 

 in the human fetus (Wolfrum, 70')- A second slight projection 

 of the mesenchyme, posterior to the above layer, is the beginning 

 of the iris. 



The membrane of Descemet increases in thickness by further 

 migration of mesenchymatous cells from the periphery, and by 

 the sixth day it forms a uniform layer throughout its extent. 

 It is now closely applied to the epithelium (fig. 35) . The spaces 



