176 M. F. GUYER AND E. A. SMITH 
greatly reduced in size. In some rabbits the opacity is evident 
without the use of the ophthalmoscope; in others, an examination 
with this instrument is necessary to disclose it. In several in- 
stances eyes which in all external respects appeared normal were 
found to have milky lenses when examined ophthalmoscopically. 
The defective lenses, however, are frequently accompanied by 
other characteristic anomalies (pls. 1 to 4). Often the abnormal 
eye has a staring look because the iris does not exhibit its normal 
reflexes and is usually more translucent than the iris of the nor- 
mal eye. The color of such an eye is peculiar; it is lavender at 
some angles and silvery at others (pl. 1). Apparently the ab- 
sence of the normal red is due in part, at least, to the clouded 
lens which keeps the reflection from the retinal blood-vessels 
from shining through. How much, if any, the retinal blood- 
vessels themselves are changed is now under investigation. Oc- 
casionally the hyaloid artery persists and a fine network of blood- 
vessels surrounds the opaque lens. While the whole lens sub- 
stance is usually clouded, in some lenses only spots are opaque. 
Not only may the lens be opaque but, as already mentioned, it 
may also be reduced in size. In such cases the eyeball, iris, and 
pupil are correspondingly small. The eyeball, for example, may 
be only one-half, one-third, or even one-fourth normal size (pl. 
1 to 4) and sunken until the eye does not extend beyond the level 
of the head (fig. 1). Again, the ball may be rotated downward 
or inward until the cornea and parts visible behind it are nearly 
out of sight. The extreme is reached in those eyes in which the 
ball collapses, leaving no trace of pupil or iris. 
Accompanying these defects are frequently a cleft iris and less 
often, a persistent hyaloid artery, due to suppression of develop- 
ment. To understand these conditions one must remember that 
in the embryo the optic cup, instead of being a complete ring, is 
interrupted on the ventral side by the choroid fissure. If this 
fissure remains open instead of closing, as it should do normally, 
the anomalies just described result. Incomplete or cleft iris is 
known as coloboma when it occurs in man. 
It is obvious that during the developmental period, especially 
from the tenth to fourteenth day when the optic cup is forming 
