62 
GEORGE E. NICHOLI-S. 
In their size and staining reactions these cells in 
P. fluviatilis are practically identical with the laige cells 
which form the “Dachkern” in the lampreys, and which also 
show evidence of a paired arrangement and have a similar 
situation on either side of the middle line. Further, the 
position and extent of the '' Dachkern ” in all vertebrates in 
which it is known to occur coincides closely with that of the 
lorward isolated portion of the neural crests of the embryo. 
In the lampreys it is true that, owing to the roof of the mid- 
brain remaining lai gely membranous, the “ Dachkern is 
restricted to the anterior poi-tiou only of that legion. 
Geotria australis. 
For the opportunity of studying the condition of Reissner’s 
fibre in this species I am indebted to Professor Dendy, not 
only for the loan of his collection of sections through the 
brain of the velasia and ammocoete stages of this lamprey, but 
also for placing at my disposal several well-preserved heads. 
In all, I have examined eleven series of sections through 
the brain of the velasia, and one series (cut transversely) of 
the ammocoete brain. 
In his paper “ On the Parietal Sense-Organs and Associ- 
ated Structures in the New Zealand Lamprey (Geotria 
australis),’^ Dendy (’07) has briefly described the condition 
of Reissner’s fibre in the brain, and its relation to the sub- 
commissural organ (‘‘ependymal grooves”). In an earlier 
paper (’02) he had described the character of the sub-com- 
missural org’an in the ammocoete. 
These paired “ ependymal grooves,” which terminate, as 
such, at the hinder end of the posterior commissure, exactly 
as they do in Petromyzon f 1 uv iati li s, nevertheless differ 
from the grooves in that species in that their shallow lumina 
are presented almost ventrally (Dendy, ’02, fig. 2) rather 
than mesially as in P. fluviatilis (figs. 84, 35, s.c.o.). 
Further, so closely, in Geotria, do these grooves approach 
each other in the middle line beneath the posterior com- 
