846 THE NERVOUS SYSTEM 



dilator muscles of the iris develop from the pigmented layer of the pars caeca. 

 Two definite layers are present in the retina, viz.: inner marginal and outer 

 mantle layers. The rudiment of the sclerotic coat of the eye is present, and 

 in front of the developing iris the mesenchyme of the sclerotic coat continues 

 below the external ectoderm forming the rudiment of the cornea. The massive 

 vitreous body is present and a delicate membrane separates the vitreous body 

 from the optic cup. It is probable that the vitreous body forms from contri- 

 butions of the optic cup and the lens vesicle. At this time, also, the rudiments 

 of the upper and lower eyelids are present as folds of the integument sur- 

 rounding the outer edges of the corneal zone of the eye (fig. 360H). 



c. Special Aspects of Eye Development 



The foregoing description of the developing eye of the chick presents the 

 common or general features of eye development. The data given below de- 

 scribe certain features of the later development of the vertebrate eye, particu- 

 larly that of the mammal and the bird. 



1) The Choroid Fissure, Hyaloid Artery, Pecten, etc. The choroid fissure 

 is the trough-like continuation of the invaginated area of the optic cup into 

 the optic stalk, and it permits a ready entrance into the optic cup. Mesenchyme 

 extends along the fissure and invades the optic cup and its developing vitreous 

 body. The central artery of the retina ,in the developing eye of the pig and 

 human also grows inward with the mesenchyme; in the region of the optic 

 cup it is called the hyaloid artery (fig. 3601). The hyaloid artery gives origin 

 to a mass of capillaries which surround but do not enter the developing lens. 

 This vascularization of the peripheral lens area persists until a short while 

 before the time of birth but regresses rapidly as birth approaches. The hyaloid 

 artery also regresses completely, leaving in its previous course a lymph space 

 known as the hyaloid canal of the vitreous body (fig. 360A). 



The choroid fissure eventually closes, including the portion which extends 

 into the region of the optic cup. In the region of the optic stalk it persists for 

 a while as a small canal containing mesenchyme and the central artery of the 

 retina. As the retina develops, the nerve fibers of the forming optic nerve 

 converge toward the optic stalk and grow inward toward the brain along the 



Fig. 360. Diagrams illustrating the development of the eye. (A) General structural 

 features of the adult mammalian eye. (Redrawn from Morris' Human Anatomy, 1943, 

 Philadelphia, Blakiston.) (B-H) Development of the eye of the chick. Ages indicated 

 on the figures. Diagram E' represents the developing eye viewed from the ventral aspect 

 showing the choroid fissure into which small capillaries are beginning to course forward 

 into the optic cup. Mesenchyme also invades the choroid fissure. In diagram H the pecten 

 has been slightly schematized. (I) Sagittal section through the developing eye of an 

 18 mm. pig embryo. Observe the hyaloid artery coursing from the optic nerve area 

 across the vitreous chamber to the lens. (T) Later stage in differentiation of the retina. 

 The rods and cones lie in the outermost area of the retina. 



