400 Charles R. Stockard. 
4 THE EMBRYOS STUDIED IN SECTION 
a Is the Origin of the Lens from the Ectoderm Dependent upon a 
Contact Stimulus from the Optic Vesicle? 
Spemann (’01) and Herbst (’01) first introduced the view that 
the lens originates from ectoderm only when a contact stimulus 
from the optic vesicle is present, although Spemann *(’07) has 
since modified his opinion for one species of frog at least. Lewis 
(04 and ’07) has found this to be true in his experiments on frog 
tadpoles, and LeCron (’07) on salamanders. King (05) claims, 
however, that such is not the case in her experiments and holds 
the view that the lens arises independently of the contact stimulus 
by the optic cup. Lewis (07) has brought objections to the 
method employed by King and so criticises her results, but the 
writer believes her method has a real advantage in that she burnt 
out the optic vesicle areas of the still open medullary tube from the 
dorsal side and thus may not always have injured the lateral 
ectoderm of the future lens-forming region. 
The writer’s experiments on the fish embryos clearly demon- 
strate that the origin of the lens from the ectoderm may be entirely 
independent of the contact stimulus of the optic vesicle. He 
continues to use the expression ‘‘contact stimulus of the optic 
vesicle” since this is what has been deemed necessary for the 
origin of the lens, although he believes that a lens may arise with- 
out any stimulus whatever from the optic vesicle either by con- 
tact or from a distance. 
In specimens lacking .optic vesicles entirely it is difficult to 
imagine that tissues are present in the brain which possess the 
power to form substances characteristically formed by optic vesicles 
and that these substances diffuse until they reach the ectoderm and 
stimulate it to formalens. In the case of isolated supernumerary 
lenses the optic cups possess lenses but still other lenses arise at 
a distance. 
Again referring to fig. 1 of plate I, a section through the eye 
region of a seventy-stx hour embryo, the ectoderm on the eyeless 
side is forming alens which is somewhat slower in development 
