Franklin Dexter 1? 
commissure has materially increased in size, and is found in its usual 
position. Anterior to this commissure a fold in the roof of the ven- 
tricle might easily be taken for the epiphysial opening, but such is not 
the case. The posterior velar arch has again changed its shape. What 
was formerly described as the horizontal arm is now distinctly ascend- 
ing and forms with the perpendicular arm a fairly acute angle, with a 
direction of upwards and forwards. The perpendicular arm has not 
altered its position, but the choroid plexus springing from it is thor- 
oughly well developed, lying in many folds, some of which have been 
cut transversely and therefore appear separated from the roof of the 
ventricle. The velum transversum is very much changed in appear- 
ance. ‘The mesenchymal tissue has thinned, its choroid fold is very 
prominent, and but for its specific position. it would be impossible to 
differentiate it from any other 
fold of the choroid plexus. 
(The breadth of this fold, as 
well as its position and rela- 
tion to the paraphysis, are 
well shown in Fig. 4.) 
The paraphysis presents a 
wonderfully regular outline, 
as well as cavity. It seems to 
be distinctly smaller than in 
the previous stage, which is 
particularly true of its cavity, 
but on the other hand, its 
walls are much thicker. A cal Golicction.” frontal series 61Z. Section TP. 
Jarge. vessel is seen in the ~<“? %#™* 
mesenchymal tissue ventrad to the epiphysis. Its position is very char- 
acteristic. It gives off branches which supply the choroid plexus of the 
third ventricle, and the vessel then divides dorsad to the paraphysis, 
and each terminal branch supplies the choroid plexuses of the lateral 
ventricles posterior to the foramina of Munro. 
Fig. 4 is a frontal section of an embryo’s brain 45 mm. It is cut 
obliquely to the cavity of the paraphysis, as seen in Fig. 3. The two 
lateral ventricles, with portions of their choroid plexuses, the cavity 
of the forebrain with its choroid folds and optic thalami on each side, 
are all easily identified. The velum stretches transversely across the 
roof of the forebrain, between the paraphysis and the choroid plexus, 
and is continuous with the mesenchyma surrounding the optie thalamus. 
It is situated dorsad to the paraphysis. The paraphysis is seen lying in 
2 
