John Warren 4 
plexus for the dorsal part, and telencephalic plexus for the ventral part, 
and I shall use these terms, as they express more clearly the exact origin 
of each plexus. 
The diencephalic plexus, D. Plx., appears as a large wedge-shaped mass 
covered by a thin layer of cells, and consisting of a loose connective tissue 
in the interstices of which numerous blood corpuscles can be seen. The 
telencephalic plexus, Yel. Plx., has the same general characteristics as 
the diencephalic. The epiphysis has become fiattened and more elon- 
gated, and is attached by a narrow stalk to the brain wall. The supra 
commissure, S.C., is seen just cephalad to the stalk of the epiphysis, 
which is prolonged forward above it. I was unable to obtain any sagittal 
series between 15 and 17.5 mm., but in a transverse series of 16.5 mm. the 
first traces of this commissure can just be made out, and therefore it 
Fie. 10. Wax model of brain of emhryo of 18 mm. Harvard Embry- 
ological Collection, Frontal Series, No. 850, X about 75 diams. 
probably appears between 16 mm. and 17 mm. as a rule, but at these early 
stages there is a good deal of variation in the development of all these 
parts. The posterior commissure is rather larger than in the previous 
stage. 
Fig. 10 is the drawing of the model of the brain of an embryo of 
18mm. This model is intended to show the circulation of the paraphysis 
at this stage. The distal end of the paraphysis, P, is surrounded by a 
venous circle, from either side of which veins, Ves., run outward and back- 
ward just caudad to the hemispheres, H, to terminate in the internal 
jugular vein, J.J. V. This vein is passing backward external to the fifth, 
V, and seventh, VJ, cranial nerves. Fig. 6 showed the intimate relation 
of the paraphysis to these vessels at 12.4 mm., and when the sections of 
this series were followed out it was found that here the vessels sur- 
rounded the tip of the paraphysis. It seems that as the paraphysis devel- 
