DEVELOPMENT OF EYE OF SPARROW 271 



which existed between this membrane and the epitheUum on 

 the one side and the lens on the other have disappeared. The 

 membrane of Descemet now measures 0.004 mm. in thickness 

 and the epithelium 0.016 mm., making a total thickness of 

 0.010 mm. The iris is still very rudimentary. 



A marked difference is noticed in the cornea at the age of 

 about seven days (fig. 36). The substantia propria (St) of the 

 cornea appears as a definite layer, which is, however, not uniform 

 in thickness. At the center of the lens it measures 0.016 mm., 

 while at the periphery it is 0.024 mm. thick. This layer is 

 apparently formed by an ingrowth of mesenchymatous cells 

 between the epithelium and the membrane of Descemet. The 

 line of junction with the epithelium doubtless represents the 

 membrana elastica anterior described by Hertwig. These cells 

 are still undifferentiated and are similar to those of the mesen- 

 chyme at the margins of the cornea. The iris is still rudimentary. 

 The margin of the original optic cup, composed of its two layers 

 (P and R), still extend to the lens. This condition has obtained 

 from the second day up to the present age. At this stage, 

 however, these two layers are relatively thinner than in the 

 earlier. The pigment layer is becoming pigmented. 



From this stage on to the adult eye about the only structural 

 changes noted in the developing cornea are an increase in the 

 thickness of the substantia propria, due to further migration of 

 mesenchymatic cells from the periphery, and a differentiation 

 of these cells into elongated spindle-like cells closely packed 

 together. Table 1 gives the modifications in the thickness of 

 the different layers of the cornea at different ages; it also gives 

 the diameters of the eye in the axial and equatorial directions. 

 This shows that the cornea increases in thickness from the second 

 day of incubation until shortly after the age of hatching, when 

 it measures 0.162 mm. in thickness. From this age on there is 

 a gradual reduction in its thickness to the adult condition, 

 when the total thickness measures 0.071 mm. This variation in 

 thickness is due mainly to modifications in thickness of the 

 substantia propria. The other layers of the cornea, the epithe- 

 Uum and the membrane of Descemet, also show slight but similar 



