ATTEMPT TO DEDUCE VERTEBRATE EYES FROM THE SKIN. 349 
‘tions of the skin shall we examine the embryological records, 
and endeavour to show that the facts are, in the main, in har- 
mony with the argument based upon structure. 
I take the pineal eye first, in spite of the disadvantage 
involved in this choice, arising out of the many debatable 
details concerning this organ. Its claim even to be an organ 
of sight at all has been disputed. Assuming it, for the present, 
to have been an eye, we are, it seems to me, justified for many 
reasons, which will be more apparent later on, in regarding it 
as having preceded in course of time the Vertebrate eyes 
proper. Its place in this discussion is, therefore, that which 
is suggested by its history as here interpreted. 
The close resemblance of the pineal organ to an eye of the 
Invertebrate (Molluscan) type was first pointed out by de 
Graaf.| The suggestion has not been accepted, for reasons 
chiefly embryological, nevertheless I am convinced that it is 
correct. If the pineal organ ever was an eye—and all the 
evidence points that way,—it should, according to our theory, 
have developed directly from the skin. With this hypothesis 
it entirely agrees, its structural relationships being precisely 
those which they should be had it arisen simply as an invagina- 
tion of a skin of the Invertebrate type, i.e. of a skin consisting 
of an external palisade layer supported internally by a layer of 
connective tissue (Diagram1). Regarding its structure alone, 
then, and this is the only point that at present concerns us, 
there is no difficulty in believing that the pineal eye deve- 
loped first as an optic pit from the skin of the ancestors of the 
Vertebrata, before that skin had assumed the Vertebrate type, 
i.e. before the palisade layer had become protected externally 
by the mucous and horny layers. 
Dealing with the development of the pineal eye more in de- 
tail,—according to the theory, the retina arose by a secondary 
multiplication of the epithelial sensory cells for the apprecia- 
tion of the variations in intensity of irritation caused by the 
movements of the pigmented granules towards the exterior ; 
while the dioptric apparatus was due to the accumulation 
1 ¢ Zool, Anz.,’ 1886. 
