58 
GEOECtE e. nichoels. 
Ill addition to the coiling thete has been an actual shrinking 
in length and a corresponding increase in thickness, for the 
fibre, which has normally in the lamprey a diameter of l'5-2 
micra, has here a diameter of 4 niicra. 
A second instance of such spiral retraction of the fibre was 
found in the same region of the spinal cord of another speci- 
men. In this the apparently functional fibre is found 
stretched taut from the sinus terminal is to a point as far 
forward as my sections go. Close against this fibre, however, 
there occurs a short stretch of freely lying fibre wound con- 
tinuously in a fairly open spiral. I find it particularly diffi- 
cult to account for the presence of this free piece of coiled 
fibre. It may of course be a remnant of the fibre broken 
some time previously, but in that case it is remarkable that 
it should have continued coiled. 
The third case is that of the lamprey, above referred to, in 
which the fibre had broken free from the sub-commissural 
organ and had retracted into a knotted end in the central 
canal. For a short distance posterior to the fi'ee knotted end 
the fibre stretches backward as a simple spirally wound thread. 
The “Dachkern.” 
This remarkable group of lai-ge and conspicuous cells has 
been known under a variety of names, having been termed 
variously the “ mesencephalic trigeminal nucleus” by Osborn 
(’88), the “ nucleus magnocellularis ” by Johnston (’01), the 
“nucleus magnocellularis tecti ” by Edinger (’01), and the 
“ mesencephalic nidulus of optic reflex cells” by Sargent (’04). 
The name “ Dachkern ” appears to have been first used by 
Kohou (’77), who applied it to the collection of large cells in 
the tectum mesencephali of Selachians, and though open, 
perhaps, to objection, is at least distinctive. 
The nucleus bas been described by many authors, and in 
nearly all classes of vertebrates. So far as I can find, hoAvever, 
it lias never been identified in mammals, while in Teleosts it 
has been, as 1 believe, erroueously (see above) homologised 
with the torus longitudinalis by Sargent (’03 a). 
