DEVELOPMENT OF EYE OF SPARROW 293 



of the developing pecten, one might consider it normal. Similar 

 folds in the retina do occur at any place and are mere artefacts 

 due to the hardening process. 



This view is also held by Froriep as true in the chick. He 

 says that the first appearance of the pecten is in the chick of 

 five days' incubation. It consists of the axial blood-vessel, 

 surrounded by mesoderm, which enters through the chorioid 

 fissure to form a flat roll in the groove formed by the closure of 

 the walls of the fissure. 



At the time of the first appearance of the pecten in the sparrow 

 the chorioid and sclerotic layers are still in a formative stage. 

 Blood-vessels can be seen in what is destined to become the 

 chorioid, but as yet no definite layer is formed. Also the meso- 

 dermal cells just outside these blood-vessels are becoming elon- 

 gated and will later form the sclerotic coat. At this stage of 

 development, therefore, there is nothing to resist the passage 

 of the mesodermal cells along the groove of the optic stalk into 

 the optic cup through the chorioid fissure. 



No differentiation of these cells in the pecten can be seen at 

 this age. They must consist, however, of cells similar to those 

 which are forming the chorioid, blood-vessels, etc., since they 

 have a common origin and closely resemble them. 



A few hours later in the developing pecten this mass of cells 

 has increased considerably in volume. In cross-section the 

 pecten now appears as a short, blunt cone. Figure 61, P, and 

 text-figure 7, P, illustrate the condition of development at the 

 age of seven and three-quarter days. The mesodermal cells are 

 extending into the conical projection. At this age all of these 

 cells are undifferentiated and resemble the typical embryonic 

 mesoderm. They are evenly distributed through the mass, 

 except at the base where they are less numerous in the center. 

 There are a few blood-vessels and blood-cells at the base, near 

 the optic disc. 



The appearance of the pecten as a cone is deceptive and is 

 due to the sectional view. As a matter of fact, it is more Uke a 

 flattened cone or ridge whose base is elongated and curved to 

 fit the inner curvature o' the eye and the closing chorioid fissure. 



JOURNAL OF MORPHOLOGY, VOL 35, NO. 2 



