S'mUCTURE AND DEVELOPMENT OF REISSNEIi’s FIBRE. 59 
In the Cyclostomes it was first recognised by Sargent (’04) 
in Petromyzon inarinus, in which species he has described 
it (op. cit., p. 154) as consisting of two groups of large cells 
placed symmetrically on either side of, and at some little dis- 
tance fi’om, the median plane, each group containing from 
eight to twelve cells. 
Sargent further claimed that he found these large cells ’in 
larval specimens of P. planeri, but, as I shall presently point 
out, it is evident that he has, in that ammocoete, altogether 
failed to interpret correctly the several structures in the roof 
of the brain, and from his figures it appears probable that 
the cells which he considers as the mesencephalic nidulus 
of optic reflex cells do not represent that nucleus at all. 
That the cells of this nucleus are, however, present in all 
the members of the Petromyzontidae is very probable, although 
possibly they become very reduced in some species. Thus 
Johnston was unable to find them in Petromyzon 
(Lampetra) wilderi (’02, p. 29) wliile I have not been 
able to identify them with certainty in ammocoetes of 
Ichthyomyzon ( E n t osp h e n u s) tridentatus even in 
comparatively large specimens (up to 95 mm.). There is, 
however, in this species, a paired group of large, rather clear 
nuclei immediately external to the sub-commissural organ, 
which quite probably represent the “ Dachkern.” 
The nucleus is present in Geotria australis, although the 
cells are not very well defined, in several of the series 
examined. The want of definition is partly due to the fading 
of the stain employed, but must be to acertain extent attributed 
to the fact that these cells appear to stain somewhat capri- 
ciously, the cytoplasm taking the stain well, in my experience, 
only when the tissue as a whole has been rather overstained. ^ 
The large cells figured and referred to by Bendy (’07, fig. 
* That this nucleus has been so long ovei'lookeJ. in the Cyclostomes 
maj’ jrerhaps be attributed in some measure to this peculiarity. I 
believe, also, that they rarely, if ever, take up the silver imjrregnation 
of Golgi, which may i)ossibly explain their alleged absence in P. 
w i 1 d e !• i . 
