240 RALPH EDWARD SHELDON 
cerebral hemispheres, and recognize that in general the hemis- 
pheres increase at the expense of the telencephalon medium as we 
ascend the phylogenetic series. For further discussion of this 
question, see Johnston (’09) and Herrick (10 b). The latter 
author, on the basis of the examination of a series of embryonic 
and adult brains of different vertebrates, has studied the method 
of evagination of the cerebral hemispheres in relation with the 
functional connections of the different parts of the neural tube 
involved in this process and has devised a schematic picture of 
the probable relations of the functional subdivisions of the neural 
tube in a primordial vertebrate whose optic and cerebral vesicles 
were still in the unevaginated condition (’10 b, fig. 72). See 
also Johnston (’11), fig. 82. 
In such an ancestral type the sulcus limitans, terminating in the 
preoptic recess, separates the ventral lamina of the neural tube 
(Bodenplatte or hypencephalic region of His) from the dorsal 
lamina (Fliigelplatte or epencephalic region). The ventral lamina 
therefore, ends in the chiasma ridge and all of the diencephalon 
and telencephalon dorsal and rostral to the sulcus limitans be- 
longs in the primary dorsal lamina, i.e., to the sensory or recep- 
tive region. The chief sensory function of this region was, in the 
telencephalon, primitively, olfaction. The tissue in the ventral 
part of this region, which lies in contact with the ventral (effer- 
ent) lamina behind, secondarily assumed the function of motor 
correlation tissue, this part being usually above fishes separated 
from the dorsal part by a sulcus, the sulcus. medius (suleus Mon- 
roi of authors), which in higher forms extends caudad from the 
interventricular foramen. By a process of further differentia- 
tion the part above the sulcus medius becomes divided into epi- 
thalamus and pars dorsalis thalami, and the part below the sulcus 
medius into pars ventralis thalami and hypothalamus, the latter 
extending forward beyond the chiasma ridge into direct continu- 
ity with the preoptic nucleus. 
The relations just described are preserved in the diencephalon 
of adult brains of many of the Ichthyopsida and are visible 
in embryos of many higher vertebrates. A transection taken 
through the rostral end of the diencephalon, accordingly, in these 
