STRUCTURE AND DEVELOPMENT OF REISSNEr’s FIBRE. 31 
Reis.) even before the posterior commissure has developed. 
Ill the later development the fibres of the posterior commis- 
sure, in making' theii' way across from the opposite sides, 
pass posterior to these axons. Such a pre-posterior commis- 
sural tract — the tractus toro-fibras Keissueris anterior — is 
found only in Teleosts (’04, pp. 198-199). 
Almost all his figures (’04), however, not only for Teleosts 
but also for Cyclostomes and Elastnobranchs, show tieissner’s 
fibre arising, either ])rincipally or entirely, from a point far 
forward beneath the posterior commissure. In explanation 
of this condition Sargent suggests that in these forms the 
anterior branch is probably derived from fibres which issue 
from cells in the habenular ganglia, and he attempts to explain, 
too, in this way, the occurrence of the fibre in blind animals 
(e.g. Myxine, Amblyopsis, ’04, p. 20G). In this manner also 
he would account for the presence in developing Squalus of a 
definite Reissner’s filire, which, he says, “ has been traced 
forward under the Schaltstiick” in embryos in which the 
“ Dachkern ” has not yet developed. 
I shall show subsequently, when describing the develop- 
ment of the fibre in embryo Amblystoma, that the early stages 
of the development of the fibre are to be seen when neither 
habenular ganglia nor “Dachkern” are distinguishable. 
Ill these early embryos, however, the elements of the sub- 
commissural organ are already well defined, and it is from the 
ependymal epithelial cells of this sub-commissural organ that 
the numerous fine fibrilhe, which join up to constitute the 
fibre, have their origin, all of these fibrillae arising 
])osterior to the developing epiphysis, and remote, 
therefore, from the habenular ganglia. I shall also point out 
that the fibre arises far forward beneath the posterior 
commissure not only in Amblystoma but in all vertebrates, 
a.mougst which, in no single instance, do I find the fibre 
arising either wholly or in part from any point morpho- 
logically posterior to the posterior commissure, notwith- 
standing that in many forms (xVmphibia, reptiles and birds) 
a well-developed “Dachkern” is present, which should 
