DEVELPOMENT OF EYE OF SPARROW 



295 



and its thickness 0.1142 mm. The cells are more closely arranged 

 forming a rather compact layer near the surface of the pecten. 

 These cells also show some granular bodies around their peri- 

 phery, which, at a later age, become pigmented and form the 

 pigment of the pecten. 



The first indication of folding into plaits is seen at the age of 

 thirteen days, or the age of hatching (text-fig. 10, P). The 

 folds appear as three sHght undulations about the middle of 

 the pecten. At this place the thickness has been somewhat 



Text fig. 9 Enlarged diagrammatic drawing of a cross-section of the devel- 

 oping pecten in the twelve-day sparrow embryo. OpN , optic nerve; P, pecten. 



Text fig. 10 Enlarged diagrammatic drawing of a cross-section of the devel- 

 oping pecten in the sparrow at the age of hatching. The first indication of folds 

 is seen. M, mesoderm; OpN, optic nerve; P, pecten. 



reduced. It is still, however, many times thicker than the fully 

 developed pecten. This condition corresponds to the ten-day 

 stage in the chick as described by Froriep. He says that the 

 pecten has become much thinner and begins to show shght 

 indications of the folds. The pectineal vein lies at the base of 

 the pecten in the optic-nerve fibers. Lieberkiihn ('72) says 

 that in the twelve-day chick embryo this vein leaves the pecten 

 at its distal end. Later in the sparrow the pecten extends 

 its base farther toward the ora serrata, so that in the adult, 

 the veins leaves the pecten slightly distal to the center of its 

 base. 



