80 
GEOUGE E. NLCIIOLLS. 
A study of tlie figures of the development of the brain of 
any lamprey will confirm these statements. 
The existence, too, of a well-developed median brain 
nucleus in the roof of the brain well behind the posterior 
commissure (Sargent, ’04, pi. i, fig. 3) is altogether in- 
credible, for, as already pointed out, the whole of this region 
of the brain-wall, at a very early stage indeed, has taken on 
its permanent character of a cubical epithelium lining the 
intra-cerebral surface of the tela choroidea. Again, 
Sargent’s description of the position of the “ optic reflex 
cells” behind the posterior commissure in the larva is 
wholly incompatible Avith the fact of the position of this 
nucleus in the adidt at a point well forward, dorsal and 
lateral to the posterior commissure. 
All these difRculties disappear when the correction which I 
have indicated above is made in the labelling of the several 
])arts shown in Sargent’s figures, and that author’s descrip- 
tions, thus amended, would agree very closely with my own 
observations recorded above. 
Sargent is thus, I believe, entirely mistaken in homo- 
logising the layer of large cells bulging downwards into 
the iter with the cells of the “D.ichkern” (his “ nidulus of 
optic reflex cells”). The cells of the “ Dachkern ” in the 
adult are not so superficial in position and are comparatively 
few in number (less than two dozen all told), Avhereas the 
cells in the group beneath the true posterior commissure iu the 
larva are veiy numerous (cf. Sargent’s figs. 1-3), and even so 
some of iheiii still aj)pear to be undergoing division. Although, 
on a casual inspection, the nuclei of these cells seem in the 
sections to form ])art of a many-layered structure, a closer 
examination reveals the fact that this appearance is mis- 
leading and is the result merely of the accommodation of the 
nuclei of a closely croAvded group of attenuated cells Avhich 
are actually arranged to form a single layer investing the 
under surface of the posterior commissure. 
From the Amntricular surface of certain of these cells a 
large cilium (or more probably a group of coalesced cilia) 
