Franklin Dexter . 15 
The midbrain is somewhat obliquely cut, but the section in the region 
of the paraphysis is nearly median. One recognizes the large cavity of 
the embryonic forebrain, with its correspondingly thin walls. 
At this stage the epiphysis (Ep) is simply an evagination of the roof, 
in front of which the posterior velar arch forms a gentle curve. Neither 
the posterior nor the superior commissure has made its appearance, nor 
as yet is there any indication of a choroid plexus. The velum (vy) is 
plainly seen between the ventral portion of the posterior velar arch 
and the paraphysis. It appears as a somewhat triangular mass of 
mesenchymal tissue protruding into the cavity of the forebrain, but 
is actually separated from that cavity by the thin ectodermic wall. It 
extends transversely across the forebrain, and so divides it, as was 
previously mentioned, into the prosencephalon and diencephalon. 
This is the earliest stage 
in which I have been able to 
identify the paraphysis. Em- 
Lryos a trifle younger, pre- 
sent in sagittal section an 
appearance which closely re- 
sembles Fig. 1, with the 
paraphysis wanting. The 
paraphysis les in the me- 
dian line, immediately dorsad 
to the foramen of Munro, 
and anterior to the velum 
transversum. At this stage 
it is a simple evagination of 
; Atetic é Fia.1. Embryo of 6.7mm. Harvard Embryological 
the brain wall, and is identi- Collection. Sagittal series 477. Section 110. x 45.2 
cal with it in structure. It “™* 
contains a large cavity which communicates with that of the forebrain. 
Fig. 2 is a most fortunate median sagittal section of a 19.5 mm. 
embryo in which the above-mentioned subdivisions of the forebrain may 
be readily identified. The posterior commissure is at this stage plainly 
visible. A well-developed epiphysis is present. This is the earliest 
stage at which I have been able to identify the superior commissure. 
It lies in its characteristic position, within the ectodermic brain wall, 
anterior to the opening of the cavity of the epiphysis. We will return 
again to this region, and will study it more closely with a higher 
power. 
It is evident on comparing Figs. 1 and 2 that the posterior velar 
arch has now totally changed its shape. In Fig. 1, it forms a curve. 
