4-44: PEOCEEDINGS OF THE AMERICAN ACADEMY, 



in it the equilibration reflex can be said scarcely to have developed as 

 yet. In this respect it is like a young lobster before the statocyst has 

 been formed (Prentiss, : 01), and its powers of orientation to gravity, 

 revealed in only a slight geotropism when at rest, are correspondingly 

 small. 



As the receptive organs for mechanical stimuli probably represent a 

 primitive stage from which the lateral-line organs and the ears of the 

 higher forms have developed, so the receptors for light doubtless give 

 some idea of what served as a source for the lateral eyes of vertebrates. 

 It has already been pointed out that the only organs that are known 

 to be light receptors in amphioxus are the eye-cups. Hesse ('98b, 

 p. 462), however, who was most instrumental in establishing this fact, 

 does not regard these organs as in any way the homologues of the 

 vertebrate eye, and in this opinion he is followed by Joseph (:04, p. 25), 

 But I must confess that to me the evidence seems to point very defi- 

 nitely to the conclusion already drawn by Boveri (:04, p. 411) that the 

 sensory cell of each eye-cup is homologous to a rod- or a cone-cell. In 

 my opinion the eye-cups of amphioxus represent a diffuse sensory 

 material from which an eye, like the lateral eye of the vertebrate, or 

 even a series of eyes, as suggested by Locy ('97), could have developed, 

 much as the ears of these animals have been differentiated from their 

 lateral-line organs. The objection to this view raised by Joseph (:04, 

 p. 24) that the photo-receptors of amphioxus do not occur in the exact 

 region from which the lateral eyes may have arisen does not appear to 

 me to be really serious. 



The steps whereby the lateral eyes have come into existence are by 

 no means easily retraced, and it is for this very reason that any indi- 

 cation such as that afforded by amphioxus is of the utmost importance. 

 Whatever has been the exact course followed by the eye in its differ- 

 entiation, two remarkable but well-recognized features have resulted ; 

 first, the retinal elements of the lateral eyes are inverted in relation to 

 the stimulus as compared with the great majority of sense organs, and, 

 secondly, the retina in vertebrates develops not directly from the 

 external ectoderm, but as an outgrowth frohi the brain. It is rather 

 striking that two investigators have published, apparently quite inde- 

 pendently, essentially the same explanation of these facts. Balfour 

 ('85, p, 508) long ago pointed out that, if we imagine that the retinal 

 part of the lateral eye was involved in the infolding that gave rise to 

 the central nervous organs, then the final positions of the rods and 

 cones at the surface of the retina away fi'om the light would be satis- 

 factorily explained, for this surface is the morphologically external 

 surface of the ectoderm. This explanation assumes that the eye was 



