28 THE ANCESTRY OF VERTEBRATES 



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 and 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 \w Amphioxus dind 

 the Ascidian larvae. Thus RAY LANKESTER( 1880) expressed 

 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 strove 

 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 Craniates to the optic organ of Ascidian larvae. 



