S86 MARION HINES 
terminalis by the angulus terminalis, as it can be done in the 
telencephalic roof plate of H173 (fig. 16, Ang. term). 
The peculiar shape of the paraphyseal arch -is more clearly 
delineated in the 19.1-mm. embryo (figs. 15,16). It. presents 
the form of a sharply elevated longitudinal ridge which here 
forms the floor of the great longitudinal fissure between the two 
cerebral hemispheres and the roof of the interventricular foramen. 
Posteriorly it is abruptly terminated by the telencephalic limb 
of the velum transversum (Vel. trans). Anteriorly it merges 
gradually with the tela chorioidea telencephali medii. The 
lateral border passes over at a sharp angle into the medial wall 
of the evaginated cerebral hemisphere (see the cross-sections, 
figs. 24, 25, 27). That portion of the hemisphere wall which 
lies contiguous to the paraphyseal arch in later stages forms the 
anterior limb of the lateral choroid plexus (see beyond, p. 87). 
The tela chorioidea telencephali medii is of variable length 
at different ages. It is defined as that portion of thetelencepha- 
lon medium which lies between the angulus terminalis (fig. 16, 
Ang. term.) and the paraphyseal arch. The choroid plexus is 
never developed in the contiguous portion of the medial wall 
of the cerebral hemisphere. 
The structures which have just been considered all belong 
in the telencephalon medium. In the adjoining part of the 
cerebral hemisphere there are two structurally distinct regions, 
ventrally the massive subcortical olfactory centers of the septum, 
and dorsally a thin area epithelialis. The area epithelialis in 
the 14-mm. embryo is structurally uniform throughout its extent, 
but morphologically it comprises two very distinct regions. The 
portion ventrally of the angulus terminalis and adjacent to the 
membranous part of the lamina terminalis is part of the septum 
1 This term is a modification of His’ angulus praethalamicus. Judging from 
figures 44 and 45 (’04), His refers to a sharp change in the direction of the midline. 
This angle is probably the same as that described in this paper, although it is 
not the anterior margin of the midthalamus region. He says that the closing 
plate of the medial hemisphere wall passes over at a sharp angle into the medial 
thalamic wall. This point may be called the angulus praethalamicus; beside it 
begins the margin of transition of the thalamic wall into the hemisphere wall, 
i.e., the margo thalamicus of the latter (p. 66). 
