348 J. B. JOHNSTON 
traced forward to the preoptic recess, apparently owing to the 
great prominence of the chiasma-ridge and the supra-optic nucleus. 
Professor Herrick has given an account of the sulci in Ichthyo- 
myzon which differs from the above account in important respects. 
With Professor Herrick’s kind permission I reproduce his figure 
73 as figure 29 of this paper. Comparison of this with my figure 
5 representing the model of the Lampetra forebrain shows that 
Herrick has given the name sulcus diencephalicus medius to a 
part of my sulcus hypothalamicus and that he has described the 
lower end of this sulcus hypothalamicus as a separate sulcus under 
the name of the sulcus diencephalicus ventralis. He recognizes 
also a suleus subhabenularis and sulcus diencephalicus dorsalis. 
As it was impossible to harmonize this description with the 
condition in Lampetra, I have secured for study at Professor 
Herrick’s suggestion the identical series of sections from which 
his drawings were made. The sections are fifteen microns thick 
and with the exception of a single broken section the series is 
perfect. A model has been made at a magnification of 100 diame- 
ters, the right half of which is drawn from the ventricular surface 
in. figure 6. The model when finished was a trifle longer than it 
should be, in the proportion of 94 to 90. 
Referring to Herrick’s figures 74 to 81, it should be noted that 
Professor Herrick viewed the sections from in front, so that the 
right side of the brain appears in the left side of his figures. Thus 
the right nucleus habenulae, which is the larger, appears on the 
left side in figures 80 and 81. From this it follows that the recon- 
struction in figure 73 (figure 29 of this paper) represents the 
ventricular surface of the right half of the brain as if it were the 
left half. This is mentioned only to show that the model figured 
is properly to be compared directly with Herrick’s reconstruction. 
The model shows the following points. The primordium hippo- 
campi is not so large as in Lampetra but has the same form and 
relations. The nucleus habenulae projects rostrad somewhat 
over the primordium hippocampi. The sulcus limitans hippo- 
‘ampi and sulcus hypothalamicus very closely resemble those in 
Lampetra. The eminentia thalami is better marked than in 
Lampetra, being a prominent ridge near the nucleus habenulae. 
