ATTEMPT TO DEDUCE VERTEBRATE EYES FROM THE SKIN. 3638 
renders any theory of the infolding of a pair of functional 
eyes into the medullary groove, to my mind at least, highly 
improbable. 
Indeed, I think it will be freely admitted that it is not easy 
to find any plausible interpretation of the embryological pro- 
cesses as recapitulatory of the actual evolution of the eye. We 
turn, therefore, to ask whether they can be better explained as 
adaptations ? 
At the outset it is obvious that the second assumption with 
which we started, that the pigmented epithelium must have 
been from the first in intimate and inseparable association with 
the retina, compels us to assume that the primary optic vesicle, 
in which the retinal half only secondarily comes in contact 
with the pigmented half, is simply a developmental adaptation. 
Let us see whether the hypothetical history of the eye above 
sketched offers any interpretation of this optic vesicle. The 
great importance of the eye in the economy of the organism, 
leads to its development earlier and earlier in the embryo, 
each shift back admitting of shorter cuts for the attainment of 
the desired end. This may well have been the rule, even when 
the eye was at the simple stage shown in Diagram II, in which 
the retina was spoon- or ladle-shaped, with the nerve represent- 
ing the handle. The most necessary requirement for the eye 
at this stage was undoubtedly the rapid formation of the 
specialised retina involving a great increase of nerve tissue, 
and its intimate association with the chromatophoral layer. 
Secondary infoldings of ectodermal tissue in order to supply 
by a short cut the great increase of sensory cells required in 
the formation of retinz is a well-known phenomenon in the 
development of other eyes, which are more obvious modifica- 
tions of the skinthan arethe Vertebrate eyes. I may mention 
the larger eyes of scorpions and spiders, the specialised retinz 
of which develop embryologically as invaginations in a plane 
the eye on each side, so as to look at the ground illuminated by the animals 
themselves, All stages occur; in some animals the eyes are only elongated 
downwards, in others a piece.of each eye has become detached so as actually 
to form a separate pair of eyes, 
