186 
long, without removing the ectoderm or injuring the intestinal canal, 
the wound rapidly heals and the brain wall is regenerated to a vari- 
able thickness but always less than normal. A small eye is often 
developed on the side removed. In other cases the retina develops 
as a part of the brain wall, and sometimes no retina at all is re- 
generated. 
The retina may certainly be regenerated in very young embryos 
after removal of its entire anlage. I expressed a different opinion in a 
previous paper‘), but with the binocular microscope I have been able 
to determine the brain region removed with great accuracy and am con- 
vinced that exactly the lateral half of the brain with the optic vesicle 
was removed from embryos which later regenerated an eye. By 
sparing the ectoderm and not injuring the intestinal canal, one is able 
to close the wound at once thus affording much more favorable con- 
ditions for the regeneration of the brain. The damage done the ad- 
jacent tissues by the heated needle used by Kine and DRAGENDORF 
may account for the non-appearance of the regeneration after com- 
plete removal of the eye-anlage in their experiments. 
The Lens. 
Embryo 121, 8,5 mm long?). In this embryo, from which the left 
half of the anterior head region was removed, the ventral part of the 
diencephalon contained a retina (see Figs. 1 and 2). The regenerated 
brain wall is everywhere thin and part of it has been transformed 
into a single layer of pigmented cells — the pigment layer of the re- 
tina. The retina proper (inner layer of the secondary optic vesicle) 
projects into the brain cavity. Immediately outside the pigment layer 
(lateral wall of the brain) and in close contact with it is a small lens 
(see Fig. 2). The position of the lens and other considerations make 
it almost certain that it was developed from the pigment layer of the 
retina (in this case also the lateral wall of the brain). This is then 
a deeply- situated reversed eye, i. e. the pigment layer is between 
the rods and cones and the surface, and the rods and cones are di- 
rected toward the surface of the body instead of toward the brain. 
The lens lies at the normally inner pole of the eye. 
1) E. T. Bert, Experimentelle Untersuchungen über die Entwicke- 
lung des Auges bei Froschembryonen. Arch. f. mikrosk. Anat., Bd. 68, 
1906. 
2) This is an embryo of Rana fusca; all the others are of Rana 
esculenta. 
