PINEAL REGION IN MAMMALIA 91 



caused confusion in studying sections as it was so similar to 

 sections through a long tubular paraphysis. However it, to- 

 gether with all the others, opens into the larger outgrowths from 

 the brain wall on the diencephalic side of the velum and is of 

 diencephalic and not of telencephalic origin. The view from 

 the inside of the brain is essentially similar to that shown in 

 figure 20 and immediately above these diverticuli the roof plate 

 runs smoothly up and back. Real plexus infoldings in the roof 

 plate do not appear until a point is reached roughly at least half 

 way to the supra-pineal recess. 



In all the specimens this tubular formation begins just at the 

 diencephalic lip of the velum and involves in all cases relatively 

 about the same extent of the velar end of the diencephalic roof 

 with quite definite limits caudally. When one takes into con- 

 sideration the distance from the velum to the superior com- 

 missure the formation as a whole is quite compact and definitely 

 limited. 



The question at once arises as to the character of this forma- 

 tion and its homologies in lower forms. It seems first of all to 

 be in the nature of a transitory affair as shown by the tendency 

 to the formation of detached and degenerating tubules and 

 vesicles. The only specimen of- an older stage available for 

 study is shown in figure 22. This is a model of the forebrain of 

 a human embryo of 80 mm. at a magnification of 20 diameters, 

 for which I am indebted to the kindness of Prof. G. L. Streeter 

 who loaned me this specimen about four years ago when the 

 model was made. The low magnification is unsuited to bring 

 out details but gives a general topographical view of the roof of 

 the forebrain. The velum, V, is very thick and dense and the 

 paraphysal arch almost entirely suppressed. On the dienceph- 

 alic side of the velum the diencephalic roof plate is prolonged 

 forward in the form of a wide pouch overhanging the roof of 

 the telencephalon to a marked extent. Irregularities are seen 

 in the lateral wall, which are apparently choroidal in character 

 or may be due to shrinkage, though the brain roof as a whole 

 was in good condition. Streeter in his account in the Keibel- 

 Mall Embryology states that the oral end of the roof plate of 



