PINEAL REGION IN MAMMALIA 89 



figures 15, 18 and 21. The paraphysal arch or telencephalic 

 roof plate is here rather sharply elevated and then bends abruptly 

 downward into the lamina terminalis, L.T"., between the median 

 walls of the hemispheres. In the depths of the model a very 

 rudimentary paraphysis, P, can just be seen. The diencephalic 

 roof is prolonged forward into a large projection a little to the 

 left of the mid-line, below which several smaller tubules can be 

 seen lying nearly side by side on either side of the mid-line. 

 Tubule number 1 represents an absolutely blind vesicle, which 

 is fused to a hollow stalk, number 2, that communicates by means 

 of a narrow opening with the brain cavity. Number 3 also 

 opens by a wide opening into the lower part of the large projec- 

 tion. Number 4 is a closed sac and is fused to the wall of num- 

 ber 5. It is almost exactly similar to number 1, while number 5, 

 like number 2, communicates with the brain by a narrow open- 

 ing. Histologically the walls of vesicles 1 and 4 are thinner 

 than those of the others and show signs of degeneration. It is 

 evident that they represent the distal ends of the original tubular 

 outgrowths which are about to become detached entirely from 

 the brain and probably will eventually disappear. 



Figure 18 is from "an embryo of 36 mm. which was in an ex- 

 cellent state of preservation and, together with the 25 mm. 

 embryo shown in figures 14, 15 and 16, represent two of the best 

 specimens in the Harvard Collection. It shows a median 

 section of this part of the brain. The velum forms here a well 

 developed fold and beginning immediately above it on the di- 

 encephalic side a large tubule partly subdivided is thrust for- 

 ward, which extends over nearly the whole of the telencephalic 

 roof plate. Above this tubule is seen the section of a larger 

 diverticulum. In figure 19 an external view of these tubules 

 is seen similar to that in figure 15. Two large diverticuli are 

 thrown out above from either side of the mid-line enclosing a 

 smaller one between them, a section through which appeared 

 in the previous figure. Below appears a number of smaller 

 and more irregular tubules lying on the telencephalic roof and 

 completely burying the paraphysis. At least two of these 

 have become blind vesicles, though still fused to the wall of a 



