TRANSMISSION OF INDUCED EYE-DEFECTS 15 
extend from the outer edge toward the pupil. The lens, which is 
large, transparent and spherical, occupies about one-half of the 
posterior chamber. In albinos such as we used the blood-vessels 
that supply the retina give to the eye a rich red color easily seen 
through the pupil and iris (pl. 1, fig. 1). 
In the embryo the optic vesicles, which arise as outgrowths of 
the ventral lateral walls of the forebrain, are well developed be- 
fore the end of the ninth day. Between the tenth and four- 
teenth days several important changes take place. ‘The ectoderm 
opposite the vesicle thickens into a disk closely connected to the 
outer wall of the latter. As the outer wall is folded in to form a 
two-layered optic cup, this disk is pulled in, eventually separat- 
ing from the ectoderm as the hollow lens vesicle which les within 
the optic cup. The cavity of the lens vesicle is gradually oblit- 
erated by the thickening of the inner wall and the lens increases 
in size by the addition of successive layers to the outside. 
The process of folding is not confined to that part of the vesicle 
in contact with the lens. The ventral wall of the vesicle and a 
part of the optic stalk are pushed in, producing a cleft or choroid 
fissure in the bottom of the cup continuous with a groove in the 
stalk. A vascular loop which later becomes the central artery of 
the retina and its hyaloid branch enters the cup through the 
choroid fissure. 
Some of the loose mesenchyme cells surrounding the optic cup 
extend around the lens to form a membrane, the posterior part 
of which is supplied by the hyaloid artery. The anterior sur- 
face receives branches of the anterior ciliary arteries. Thus, by 
the thirteenth day, the lens is relatively large and is surrounded 
by a membrane richly supplied with blood-vessels. This vas- 
cular membrane is an embryonic structure which serves for the 
nutrition of the lens during its growth. It disappears before 
birth. 
THE DEFECTIVE EYES . 
The chief defect, common to all the eyes where there is suf- 
ficient eyeball left to permit of internal examination, centers in 
the lens. It is always opaque, in whole or in part, and it may be 
