90 JOHN WARREN 



neighboring tubule. Their walls are thinner than those of the 

 others and are e\adently about to separate off and probably 

 degenerate. Figure 20 gives a view of the same model seen from 

 the inside of the brain. The velum, V, forms a well marked 

 ridge, in front pi which a slight depression in the telencephalic 

 roof plate marks the former opening of the paraphysis. On the 

 diencephalic side of the velum the roof plate shows two large 

 irregular openings separated by an irregular ridge in the mid- 

 line, the one on the right being much larger than the one on the 

 left. In the depths of these larger diverticuli are seen the 

 openings of the smaller tubules, the majority of which com- 

 municate with the right diverticulum. It can clearly be seen 

 that all of these represent outgrowths or evaginations from 

 the roof plate and differ in this respect from the usual invagina- 

 tions or ingrowths of the brain wall found in plexus formations. 

 Above the larger prolongations the diencephalic roof plate sweeps 

 gradually upward with only a slight median depression. In the 

 depths of these tubules were two or three closed vesicles either 

 entirely cut off from the brain wall or still partly fused and 

 showing the same signs of degeneration as was seen in the 31 

 mm. embryo (fig. 17). An embryo of 37 m^n., H. E. C. no. 820, 

 has a much simpler arrangement in this region. The evagina- 

 tions from the diencephalic wall formed three fairly large pouches, 

 one in the mid-line with two smaller ones on either side, and 

 there was no sign of any smaller tubules or detached vesicles, 

 while in an embryo of 40 mm., H. E. C. no. 1917, there is a 

 simple medial pouch pushed forward over the telencephalic 

 roof plate with a very few tiny outgrowths from it. An embryo 

 of 42 mm., H. E. C. no. 838, showed practically the same condi- 

 tions as in the 36 mm. embryo (fig. 19), and in the oldest em- 

 bryo in the collection which is shown in figure 21. There was in 

 this embryo, however, some little shrinkage in the walls of the 

 postvelar tubules, which must have caused some slight distor- 

 tion, but the general plan however is essentially similar to that 

 shown in figure 19. There is one especially prominent tubule 

 appearing below and in the mid-line. The other specimens 

 almost always had some similarly placed tubule which at first 



