1532 A MANUAL OF ANATOMY 



ment being called the optic vesicle. The remainder of the protrusion consti- 

 tutes a narrow, hollow pedicle, called the optic stalk, which is attached to the 

 wall of the diencephalon, the cavity of the stalk communicating with the 

 third ventricle. The enlargement formed by the optic vesicle lies beneath 

 the ectoderm of the lateral surface of the head. 



The ectoderm corresponding to the optic vesicle becomes thickened and 

 depressed, this depressed portion constituting the lens-area. The depressed 

 ectoderm is deepened and converted into a kind- of cup, and, the mouth of 

 the fossa becoming constricted, its lips unite. In this manner the lens-area 

 becomes transformed into a closed ectodermic sac, called the lens-vesicle, 

 from which the crystalhne lens is differentiated. The lens-vesicle now becomes 

 completely separated from the surface-ectoderm, with which it was originally 

 continuous. 



Coincident with the formation of the lens- vesicle, an important change 

 takes place in the optic vesicle. The distal or outer wall of the optic vesicle 

 is invaginated or pushed inwards towards the inner wall until the two walls 

 come into contact, this being effected by the lens-vesicle. The optic vesicle 

 thereby becomes transformed into the optic cup, the wall of which is com- 

 posed of two layers — outer and inner. The lens-vesicle lies just within the 

 mouth of the cup. The portion of the cup close to the margin represents 

 the ciliary region, and the fundus of the cup forms the retinal region. The 

 cup becomes surrounded by mesodermic tissue. 



During the formation of the optic cup it is not only the distal or outer 

 wall of the optic vesicle which is invaginated, or pushed inwards, by the 



Lens Pit Lens 



Ectoderm | Optic Vesicle ; Optic Cup 



Optic Stalk - Re n- 1^6 K^ /-^ • o 11 



J|&^ Pigmentary i M-^ Optic Stalk 



H^ ^tffi Layer of ' ^ " 



Optic Cup Retinal Layer 

 of Optic Cup 



Fig. 642. — Development of Crystalline Lens and Optic Vesicle. 



lens-vesicle. The ventral wall of the optic vesicle, and the ventral wall of 

 the part of the optic stalk, which is adjacent to the optic vesicle, are also 

 invaginated. As the result of this extra invagination a cleft, of temporary 

 duration, is left on the under surface of the optic cup, which is called the 

 choroidal fissure. Through this fissure mesoderm enters the optic cup, 

 posterior to the lens-vesicle. In this mesoderm a nutrient artery is developed, 

 called the hyaloid artery, which furnishes the minute vessels conferring vas- 

 cularity upon the capsule around the developing lens. 



As a rule the choroidal fissure becomes closed, and then the hyaloid artery 

 is enclosed within the cavity of the optic stalk, and constitutes the arteria 

 centralis retinse. 



In cases of non-closure of the choroidal fissure, the region of the lissure 

 remains unpigmented, and one of two congenital deficiencies in the eye are 

 met with, each being known by the general term coloboma. If the patent 

 lissure affects the ventral wall of the optic cup, then the deficiency in pigment 

 affects the choroid, and the condition is known as coloboma choroidea. If 

 the patent fissure affects the lower margin of the optic cup, then the deficiency 

 affects the lower part of the iris, and the condition is known as coloboma 

 iridis. 



As stated, the wall of the optic cup consists of two layers. The outer 

 layer, which is comparatively simple, gives rise to the pigmentary layer ol the 

 Tetina. The inner layer is, on the other hand, very complicated. After 

 much differentiation it gives rise to all the other layers ol the retina. The 



