STRUCTUIU^ AND DKVELOPMENT OF REISSNEr’s FIBRE. 77 
Petroniyzon planevi (?) we Lave apparently a progres- 
sively arranged series in the evolution of the sub-commissural 
organ. 
I have further shown that in all the members of this family 
that I have examined this organ ends abruptly at the hinder 
border of the posterior commissure, or quickly fades away 
behind it into the general ependymal epithelium of the iter. 
For Petro my zon mariuus, however, Sargent has given 
a widely different accoujit (’04), stating that the paired 
grooves in that species curve around the hinder border of the 
posterior commissure and pass dorsally (cf. his text-figure a, 
p. 151) to the anterior extremity of the mesocoel, but, as 
already remarked, his oilier figures (PI. 1, figs. 0 and 1 ) do 
not at all bear out his statements. I have not myself 
examined sections thiongh the brainof this jiarticular lampre}', 
but assuming that the figures given by Sterzi (’07) represent 
correctly the extent of the choroid plexus of the mid-brain 
and its relations to the jiosterior commissure, it is obvious 
that the entire dorsal surface of the hitter structure lies ventral 
to this clioroid plexus. That being so, the space overlying 
the posterior commissure, as I'cpresented by Sargent in his 
fig. 7, can only be part of the forward extension of the 
mesocoel. The two halves of the sub-commissural organ 
(ependymal grooves) should, therefore, ajipear in the figure 
upon the dorsal surface of the posterior commissure if, as 
Sargent states (’04_ p. 152), “ Posteriorly both grooves extend 
downward under the jiosterior commissure, at the 
same time coming nearer together, litre they curve around 
the commissure . . . and continue cephalad into the 
recessus of the mesocoel above the commissure and thence 
into its anterior honis. 'I'lie horns of the recessus are com- 
])letely lined by this characteristic ependyma. A transverse 
section through the anteidor part of the postei'ior commissure 
shows the horns of the recessus as small circular orifices witli 
thick walls composed of this radiating ependyma (pi. i, fig. 6, 
rec. 1').” 
The last part of this statement is especially difficult to 
