SENSE OR RECEPTOR ORGANS 849 



vessels. Thus it happens that in addition to the entrance of mesenchyme and 

 blood vessels into the vitreous chamber the pigmented and nervous layers of 

 the pars optica retinae fold inward around and enclose the blood vessels and 

 mesenchyme. This forward projection of ectodermal and mesodermal tissues 

 into the vitreous chamber toward the lens forms the pecten of the bird's eye 

 (fig. 360H). From the seventh to the eleventh days the rudiment of the pecten 

 increases greatly in length, and becomes very narrow, folded, and comb- 

 shaped. Shortly before hatching, the number of folds increases to about 18, 

 and the structure as a whole is highly pigmented and vascularized. The pecten 

 appears to increase the vascular supply to the vitreous chamber, and also, it 

 is possible that the pecten may act in some way to increase the visual powers 

 of the retina. In the reptiles a similar, but less complex projection, the vascular 

 papillary cone, is developed, and in the eye of teleost fishes the falciform 

 process may be homologous with the papillary cone of reptiles and pecten 

 of birds. 



2) The Formation of the Lens. The early formation of the lens vesicle 

 from the overlying ectoderm appears to be dependent upon inductive influences 

 emanating from the optic vesicle in some species, e.g., Bombinator, but in 

 others, e.g., Rana esculenta, the lens vesicle appears to form independently. 

 (See Werber, '16, and Spemann, '38, Chapter 3.) The inner wall of the lens 

 vesicle differentiates into elongated slender cells of the lens. The nuclei remain 

 near the center of these slender cells, and the cells gradually transform into 

 the transparent lens fibers. The outer, lateral wall of the lens vesicle forms 

 a layer of low columnar cells, the lens epithelium (fig. 360H and I). 



3) The Choroid and Sclerotic Coat of the Eyeball; the Cornea. The devel- 

 oping optic cup is at all times surrounded by mesenchyme. This mesenchyme 

 condenses around the pigmented layer of the optic cup to form two distinct 

 layers, namely, ( 1 ) An inner vascular coat immediately surrounding the pig- 

 mented layer, and (2) an outer white fibrous thick connective tissue layer. 

 The inner vascular coat forms the soft, vascular choroid coat of the eyeball, 

 whereas the fibrous layer develops the hardened sclera or sclerotic coat. The 

 sclerotic coat in reality is the skeletal investment of the eyeball, upon which 

 the extrinsic muscles of the eye insert, and from which internally the ciliary 

 muscles or muscles of accommodation take their origin (fig. 360A). Also, 

 the muscles of the iris indirectly are dependent upon the sclera for their effi- 

 ciency. The choroid coat is the main source of blood supply for the eyeball 

 as a whole. It is highly pigmented and absorbs excess light rays from the 

 retina. In many vertebrates, including various mammals such as the cat, dog, 

 cow, deer, ferret, etc., the inner layer of the choroid coat near the retina de- 

 velops a light reflecting surface, the tapetum lucidum. In the cat and other 

 carnivores, this reflecting surface appears to be due to crystals of guanine, 

 while in the cow it is due, probably, to connective tissue fibers which glisten 

 and thus reflect the light. 



