320 JOHN WARREN 



this part of the roof has grown steadily upward and backward 

 so as to form a large sac-like enlargement, and the distance 

 between the velum and the opening of the epiphysis has contin- 

 ually increased. After this stage we shall find a tendency for 

 these two points to approch each other, causing very striking 

 alterations in the shape and relations of the various parts of the 

 roof of the diencephalon. As an apparent result of this exces- 

 sive enlargement of the hinder part of the post velar arch, the 

 epiphysis has been pushed somewhat backward. It forms a long, 

 hollow tube with still a very narrow communication with the 

 cavity of the diencephalon. The pineal eye has shifted dorsally 

 and is still almost in contact with the tip of the epiphysis, but is 

 separated by a considerable interval from the top of the post 

 velar arch. There is present a new structure — the superior com- 

 missure, S. C. — which appears considerably later than the pos- 

 terior. It lies in its characteristic position immediately anterior 

 to the opening of the epiphysis, which separates it from the pos- 

 terior commissure. The latter is greatly increased in size and is 

 in contact with the posterior aspect of the epiphysis, occupying 

 now all of that segment of the diencephalic roof caudad to the 

 epiphysis. It has also extended backward well into the mid 

 brain and its hinder end is so blended with the outer layers of the 

 wall that one cannot well give it any definite limit. Noteworthy 

 is the marked increase in the size of the mid brain, the roof of 

 which now tends to roll somewhat forward toward the epiphysis. 

 As a result of this, and also of the backward development of the 

 post velar arch, the posterior commissure is somewhat compressed, 

 and an angle is formed at about its middle, dividing it in a general 

 way, into an anterior part in relation to the diencephalon and a 

 posterior part in relation to the mid brain. 



Fig. 11 shows the brain of Lacerta muralis of 26 mm. Here 

 the paraphysis is larger, its outline is rather irregular and it is 

 growing farther backward toward the epiphysis and pineal eye. 

 The relation of the organ to the surrounding veins is well shown in 

 the model. A large vessel — the superior sagittal sinus, S. S. S. 

 — lies between the two hemispheres. The paraphysis as it devel- 

 ops grows into the small veins which are the anlagen of this big 



