78 
GKOKGE E. NIOnOLI,S. 
reconcile with his tig. 6 (pi. i), for he there shows this recess 
lying beneath the posterior coinniissure instead of above 
it, as described. 
Not only are Sargent’s statements and figures thus con- 
flicting', but I find nothing in any of the lampreys which I 
have examined which will throw any light upon the condition 
which Sargent figures (pi. i, fig. 6) — nothing, that is, of the 
nature of a postero-dorsal continuation of the sub-com- 
niissural organ behind and above the posterior commissure. 
1 have, however, described paired “diacoelic recesses ” (see 
above) which are shallow pockets bulging caudally from 
the recessus infrapineal is to overlie slightly the 
posterior commissure from in front (fig. 35). These 
recesses are completely lined with the characteristic epithe- 
lium of the sub-commissural organ, and in transverse sections 
through the anterior border of the posterior commissure do 
]iresent an appearance not unlike that figured by Sargent for 
the mesocoelic recess. These diacoeiic recesses of Petro- 
myzon fluviatilis are almost certainly identical with the 
“ recessi postabenulari ” of Sterzi, which are apparently 
particuhu'ly well develo})ed in P. marinus. An explanation, 
therefore, which suggests itself in connection with this conflict 
of statement and figure in Sargent’s paper, is that that author 
may have mistaken the diacoeiic recess opening forwardly 
into the infrapineal I'ecess for a mesocoelic recess opening 
backwardly beneath the choroid plexus of the mid-brain. An 
accidental misplacement of a few sections might easily give 
rise to such a confusion. 
(b) Development of the Sub-commissural Organ 
and Reissner’s Fibre. 
Sargent’s account, too, of the development of the fibre is 
incorrect, which I can only suppose to be due to the fact that 
he did not recognise the embryonic sub-commissural organ, 
having failed once again to correctly identify the posterior 
commissure. 
A comparison of Sargent’s figures of the brain of the 
