PINEAL REGION IN MAMMALIA 87 



figure 29, where the velum is incorrectly placed and what is 

 labelled paraphysis should really be a postvelar outgrowth from 

 the diencephalon. 



Figure 18 is from a model of a median section of the forebrain 

 of the 36 mm. embryo similar to figure 14. The velum corre- 

 sponds to that in the 25 mm. embryo and in front of it a rela- 

 tively small paraphysis, P, projects forward from the telen- 

 cephalic roof plate." It is connected to the telencephalon by a 

 solid stalk with a slight depression indicating the original open- 

 ing and a small cavity persists in the distal end only. The 

 paraphysis and telencephalic roof plate are completely buried 

 under the mass of tubules which spring from the postvelar arch 

 and extend forward. In three embryos of 37, 40, and 42 mm. 

 respectively, all in good preservation, there was no sign of any 

 sort of a paraphysis. An embryo of 44.3 mm., the oldest em- 

 bryo in the collection, is represented in figure 21, which gives 

 an external \dew of the roof of the telencephalon and dienceph- 

 alon similar to figures 15 and 19. Here a tiny paraphysis, P, 

 can be seen in the midline of the telencephalic roof buried under 

 the mass of diencephalic tubules. It extended through a very 

 few sections, only one of which had a cavity, and is attached by 

 a thin solid stalk to the brain. The paraphysis in this specimen 

 is about to degenerate and entirely disappear. 



Of all the embryos studied, excluding doubtful cases and 

 slightly damaged specimens, a more or less rudimentary para- 

 physis could be found in only eight instances. It therefore may 

 be said to exist in human embryos as a very inconstant and 

 variable structure. 



S. Postvelar arch and postvelar tubules or diverticuli 



The earliest sign of this peculiar modification of the velar 

 end of the postvelar arch appears in embryos of from 19 to 23 

 mm. In figure 13, an embryo of 23 mm., it forms a simple 

 transverse fold somewhat tent-shaped in the mid-line. This 

 was earlier mistaken by the author for a paraphysis, as the real 

 paraphysis was here so small that it was at first overlooked. 



