514 



Posteriorly they terminate at the hinder margin of the posterior 

 commissure. Immediately behind this point the choroid plexus of the 

 mid-brain dips down into the aquaeductus Sylvii, or iter, in the shape 

 of a thin vertical lamella, which forms a highly vascular longitudinal 

 septum dividing the upper part of the iter into right and left halves 

 (fig. 3), The anterior portion of this septum is visible in sections 

 beneath the ciliated grooves in the region of the posterior commissure 

 (fig. I), while it extends backwards 

 throughout the whole or very nearly 

 the whole length of the mid-brain. 



Thanks to the kindness of Dr. 

 S. J. Hick s on, F. R. S. Professor 



Fis:. 4. 



Fig. ."1 Transverse section of the same specimen just behind the posterior com- 

 missure, showing the ingrowth of the choroid plexus of the 

 mid-brain to form avertical septum. 

 Fig. 4. Transverse section of the brain of the Owens College Ammocoete through 

 the region of the posterior commissure. 



of Zoology in the Owens College, I have been able in a striking 

 manner to confirm the results thus obtained in the New Zealand 

 Ammocoete by an investigation of the corresponding organs in one of 

 the European species. Professor Hickson kindly placed at my disposal 

 for the purposes of this investigation a series of transverse sections, 

 which had been cut a short while before by his assistant, but which 

 he had not yet examined, and I was delighted to find in these, with- 

 out the slightest difficulty, the structures which I had previously 

 discovered in New^ Zealand. 



