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MUSEUM OF COMPARATIVE ZOOLOGY. 7i 
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 zeigen 
die Zellen der Retina ein eigenthiimliches Verhalten. Ihre Kerne werden 
sehr langgestreckt und sie ordnen sich um den Nerv in 1-2 dichten Lagen 
dergestalt an, dass sie schon kurz vor defn Eintritt des Opticus in die 
aiissere Kérnerschicht und auf der ganzen Strecke, die derselbe sich durch 
die Kérnerschichten hinzieht, eine Art fester Rohre 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 Myxinide, it should be said that, from the investigations of 
J. Miiller (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 different 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. 
Miiller ("74, p. 14, and Fig. 3, Taf. XI.) recognizes in it, however, the 
