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their origin not from the epidermis, like other sense-organs, 
but from the wall of the brain vesicle: In the brain vesicle 
the larvae of the Ascidians possess a little eye, provided 
with a lens. In  Amphioxus we find in the anterior wall 
of the brain vesicle only a little median pigment-spot. A 
number of similar pigment-spots occur also in the wall of 
the whole medullary tube with the exception of a few anterior 
segments. No transitions, however, are found between these 
extremely simple organs gn the highly complicate Vertebrate 
eye, which is built on a wholly different plan and to the 
composition of which not only the brainwall but also the 
bodywall (lens) and the mesoderm contribute “Fertig, wie 
Athene aus dem Haupte des Zeus”, FRORIEP (1906, p. 140) 
says, “tritt das Vertebratenauge in die Erscheinung” and 
from the evidently more or less degenerate eyes of Cyclo- 
stomes onwards no other organ in the phyletic series of the 
Craniates exhibits such a uniformity in the essential features 
of its organisation. 
Attempts to derive the Craniate eyes from them. — Yet 
attempts have not been wanting to trace back the eyes of 
Craniates to the corresponding structures in Amphioxus and 
the Ascidian larvae. Thus RAY LANKESTER (1880) expresse 
as his conviction: “that the original Vertebrate must have 
been a transparent animal, and had an eye or pair of eyes 
inside its brain, like that of the Ascidian tadpole. As the 
tissues of this ancestral Vertebrate grew denser and more 
opaque, the eye-bearing part of the brain was forced by 
natural selection to grow outwards towards the surface, 
in order that it might still be in a position to receive the 
influence of the sun’s rays”. BALFOUR (1881, p. 419) points 
to another possibility, viz: that the eye of the Ascidians is 
a degenerate form of the Vertebrate eye. 
JELGERSMA (1906) traces in details the way in which the 
transformation of the Ascidian optic organ into the Verte- 
brate eye could have been performed. According to FRORIEP 
(1906 b), however, the Craniate eye cannot be derived 
directly from that of the Ascidian larva but both have devel- 
oped from an original condition in which two eye-pits 
were lying at the surface. After the involution they strave 
to regain the light. In the Ascidians one was lost and the 
other applied itself closely to the transparent body-wall. 
WILLEY (1894), on the other hand, homologizes the parietal 
eye of Granaten: to the optic organ of Ascidian larvae. 
