366 J. B. JOHNSTON 
The roof plate and floor plate converge into the lamina terminalis, 
where of course they end. The four massive columns on each side con- 
verge into the interventricular foramen, and in larvae with wide fora- 
mina and adult urodeles they may be followed through the foramina 
into the evaginated hemispheres. Bearing in mind the fact that during 
development the roof plate and floor plate retain permanently their 
primitive attachments to the lamina terminalis, and that it is only the 
massive lateral columns which are evaginated into the hemispheres, it 
clearly follows that these columns of the diencephalon are continued into 
the hemispheres in the form shown by the accompanying diagram (fig. 
84), the zona limitans lateralis representing the locus of the sulcus medius 
and the zona limiants medialis the line of union of the dorsal and ventral 
columns in the lateral evaginations rostral to the fusion of the roof plate 
and floor plate in the lamina terminalis. 
It has been noted elsewhere (’11 b, p. 540) that the assignment of 
the lamina terminalis in this paragraph to the floor plate was an 
error of inadvertance. It has been shown also (711 b, pp. 534, 
535 and in this paper) that the sulcus diencephalicus medius has 
no relation with the interventricular foramen or the zona limitans 
lateralis either in amphibians, fishes or cyclostomes. 
The conception of the folding over of the lateral walls of the 
diencephalon illustrated in Professor Herrick’s figures 83 and 84, 
such that the dorsal and ventral borders fuse in the medial zonae 
limitantes to form two tubes extending forward (the hemispheres), 
seems to the writer to be without basis in fact. 
In figure 72 Herrick (10) has given a diagram of the forebrain 
in a hypothetical vertebrate ancestor. In this the representation 
of the sulcus dorsalis issubject to the criticisms made above (p. 352). 
No ground is given for the assumption that the retinal area does 
not involve the dorsal border of the brain, except the erroneous 
supposition that there is no di-telencephalic fissure in cyclostomes. 
The suleus medius is represented as a horizontal sulcus ex- 
tending caudad from the site of the interventricular foramen, 
whereas in all lower vertebrates it runs nearly vertically up into 
the dorsal sac. The sulcus ventralis should lead to the site of 
the interventricular foramen as it does in cyclostomes, selachians 
and amphibians. The terminal ridge is represented as an inde- 
pendent thickening rostral to the chiasma ridge, whereas the ter- 
minal ridge in all vertebrate embryos itself becomes the bed for 
the optic chiasma (Johnston ’09). 
