DEVELOPMENT OF EYE OF SPARROW 279 



the lens is, therefore, not confined to the region of the ectoderm 

 where normal development occurs, but to any area with which 

 the optic vesicle comes in contact. Froriep ('06) concludes that 

 the formation of the optic cup is due not so much to the invagi- 

 nation, but to a more rapid outgrowth of the rim. 



At this age there is no space between the invaginating epithe- 

 lium and the anterior wall of the optic cup. A slight remnant of 

 the cavity of the optic vesicle is still seen between what is now the 

 outer and the inner (invaginated) layers of the optic cup. As 

 the edges of the pit approach each other, a space appears between 

 the forming lens and the inner layer of the. optic cup. This is 

 due to the ingrowth of mesenchymatous cells and the formation 

 of the vitreous body to be described later. By the complete 

 growing together of the edges of this pit the lens vesicle is formed 

 (text figure 6, C, and figure 45, Lc). This occurs in the chick at 

 about sixty-four hours' incubation. Froriep ('06) says that it 

 varies between sixty-two and seventy-three hours. The shape 

 of the lens vesicle is now almost a sphere. The axial and equa- 

 torial diameters are 0.128 mm. and 0.144 mm., respectively. 



After the complete closure of the lens vesicle, it remains in 

 cellular contact with the epithelium for a short time. This 

 connection very soon disappears and the epithelium extends 

 across and immediately in front of the vesicle in a single layer of 

 cells which later becomes the conjunctival epithelium (text 

 fig. 6, C). 



At this stage the wall of the lens vesicle in the chick is almost 

 uniform in thickness. A slight difference, however, is noticed. 

 The thickness of the anterior wall is 0.04 mm. and that of the 

 posterior wall is 0.055 mm. (Froriep). In the pig at this stage 

 of lens development I find the anterior wall is decidedly thinner, 

 measuring 0.04 mm., while the posterior measures 0.072 mm. 



About this time, according to Keibel and Mall ('12), the lens 

 capsule is seen. They say it is developed from the cells of the 

 lens and not from the surrounding mesoderm. 



In the 6-mm. pig embryo, which represents approximately 

 this stage, I find a stream of mesenchymatic cells migrating 

 into the optic cup between its edge and the lens vesicle. The 



