MUSEUM OF COMI'AK.VI'IVK ZOOl.OCJY. 77 



the retina, such as occurs in Typhlogobius. But it is interesting to no- 

 tice in this connection Kohl's description of this portion of the nerve in 

 Proteus. He ('89, p. 408) writes : " Beim Durchgang des Opticus zeigeu 

 die Zellen der Retina ein eigenthlimliches Verhalteu. Ihre Kerne werden 

 sehr langgestreckt und sie ordnen sich ura den Nerv in 1-2 dichten Lagen 

 dergestalt an, dass sie schon kurz vor dem Eintritt des Opticus in die 

 aiissere Kornerschicht und auf derganzen Strecke, die derselbe sich durch 

 die Kornerschichten hinzieht, eine Art fester Eohre um ihre bilden." 

 It is quite possible that the pigment sheath described in the Typhlogo- 

 bius eye may have been preceded by such a cellular sheath as this ; but 

 if so, my conjecture that it is derived from the nerve itself, and not from 

 the surrounding retina, would be, of course, erroneous. It is also worth 

 mentioning that Berger ('81, p. 262) has described pigmented fibres 

 arising from the choroid as passing through the optic nerve in some 

 fishes. 



We have not yet sufficient knowledge of the minute structure of the 

 eye of any of the other blind vertebrates, Myxine and its allies excepted, 

 to make possible further detailed comparison. With reference to the 

 eyes of the Myxinidte, it should be said that, from the investigations of 

 J. Mliller ('35-41), and, later, W. Miiller ('74, pp. 7-15), we know that 

 they are far more rudimentary than in any other vertebrate whatever, 

 unless we admit the exceedingly problematical pigment spot at the 

 anterior end of the nerve cord of Amphioxus to be homologous with the 

 eye. It is, however, instructive to notice wherein the eyes that we have 

 been considering may be regarded as passing along the same degenera- 

 tive road over which the Myxinoid eye has passed, and in what respects 

 they might seem to be on diff"erent roads. The eye of Myxine is buried 

 in the tissue of the head in much the same way as in the other forms, 

 excepting that, . in addition to the layers of skin and the connective 

 tissue by which it is covered, there is also a layer of muscle over it, and 

 it is immediately surrounded by a sort of capsule containing in its sub- 

 stance much fat. As the foregoing pages have shown, there is no indi- 

 cation of either the muscle layer over the eye or the fatty layer around 

 it in Typhlogobius, Proteus, or Talpa ; but it is of course entirely beyond 

 our power to say that there never could be such structures. 



Neither lens nor eye muscles, nor anything that can properly be re- 

 garded as a cornea, sclerotic, or iris, are present in the eye of Myxine- 

 The primitive optic vesicle never becomes wholly obliterated, and the 

 retina reaches only a very rudimentary degree of differentiation. W. 

 MiiUer ('74, p. 14, and Fig. 3, Taf. XL) recognizes in it, however, the 



