DEVELOPMENT OF EYE OF SPARROW 313 



these portions of the retina will expend a greater amount of 

 energy. A richer blood supply is therefore necessary. This 

 adaptation also indicates that it is very probable that the sparrow 

 has binocular vision with the temporal regions of the retina. 

 It would require but a slight convergence of the eyes to accom- 

 plish this. Experiments made with sparrows with one eye 

 extirpated, prove that the bird can easily move its eyes suffi- 

 ciently to bring about binocular vision to within 1 mm. of the 

 fovea. Many birds have a second fovea located in this temporal 

 region which isused in binocular vision (Slonaker, '97 and '18). 

 The fact that the area and fovea centralis do not begin to show 

 development in the sparrow until about the age of hatching, 

 and that the most rapid differentiation occurs after the eyes 

 open, indicates that the light stimulus may exert a great influence 

 in its formation. In this connection it is interesting to note 

 that Held ('96) has found in animals born with their eyes closed 

 the stimulus of light hastens the development of myelination 

 in the optic-nerve fibers; 



THE MUSCLES OF THE EYE 



Extrinsic Jiiusdes. The development of the extrinsic muscles 

 begins at a much earlier age than that of the intrinsic muscles. 

 At the age of three days' incubation some of the mesoderm, in 

 regions at the posterior part of the orbit, begins to group itself 

 into very indistinct masses of more closely packed cells. They 

 are more condensed near the optic-nerve entrance and grow 

 less dense and distinct as they are traced toward the eyeball. 



By the fifth day these masses are more sharply marked off 

 from the rather loosely arranged surrounding cells. The cells 

 composing these masses have become slightly spindle-shaped 

 in outline. They are arranged with their long axes parallel to 

 the greatest dimension of the mass and in general parallel to the 

 surface of the eye. These changes continue rapidly, so that 

 by the seventh day the elongation of these cells and their parallel 

 arrangement is easily demonstrated (fig. 56, M). The cells 

 at the distal ends of these masses gradually blend with the cells 

 forming the sclerotic coat. 



