159 
completely enclose the lens in a part of its extent — a fact which 
shows almost conclusively that the lens was developed from this retinal 
layer. In the greater part of its extent the lens is crowded between 
the retina proper and the pigment layer, there being no rods and 
cones developed at the area of contact. One end of the lens projects 
free into the connective tissue above. 
Embryo 332, 3,6 mm long. This embryo was also operated as 314. 
It was killed nine days after the operation. The eye remained re- 
versed, the normally outer pole is partly fused to the brain and the 
inner pole is very close to the surface of the body. In the cavity of 
the secondary optic vesicle is a large lens developed apparently from 
its margin. In the outer pigment layer very near the ectoderm is a 
small spherical evagination. This structure is not far enough developed 
to enable one to say that it is a lens, but it is probably an early stage 
of lens formation inasmuch as we must suppose that the lenses in 
embryos 121 and 314 passed through a similar stage in their early 
development. 
In the pigment layer of the same eye not far from the lens-like 
evagination is a biconvex thickening which can only be interpreted as 
a simple refractive organ. It is very similar to the ectodermal lens 
of some insects. 
Embryo 367, 3,2 mm long. The brain was opened in the dorsal 
median line and the right optic vesicle of another embryo of about the 
same size was pressed completely inside. It was killed the sixth day 
after the operation. No abnormality was visible externally. The se- 
condary optic vesicle was in the brain forming a small part of its roof, 
and with its cavity directed dorsally. A mass of brain tissue projects 
dorsal to the optic vesicle and a lens is formed in the cavity of the 
optic vesicle from this brain tissue. The structure of the lens and the 
way in which it is connected to the brain tissue leaves no room for 
doubt as to its origin. 
In several other embryos in which the optic vesicle was trans- 
planted it is very probable that the lens was formed from brain tissue; 
but there is some uncertainty 'in these cases because either of ad- 
hesions between the lens and parts of the optic vesicle or to the lens’ 
being in too early a stage of development to be positively identified 
as such. 
Embryo 345, 3 mm long. The right optic vesicle was cut off, re- 
versed and pushed into the brain after enlarging the opening made by 
removing it. Its ventricular opening was directed outwards. It was 
killed the fifth day after the operation. The brain had regenerated 
