274 SCOTT. ' [Vol. I. 



not likely, as only one cranial nerve, the third, has its origin in 

 this region. 



The cerebellum is formed, as Ahlborn has shown, by the 

 appearance of fibres in the posterior wall of the already-men- 

 tioned dorsal fold, which marks the junction of the mid and 

 hind-brains. This fold is found in the embryo, but I have not 

 detected the presence of fibres in larvae of less than 20 mm. 

 length. This portion of the roof of the fourth ventricle (for 

 this fold is morphologically, and at a later stage actually, a part 

 of the roof) is, of course, not included in the thinning which 

 accompanies the formation of the choroid plexus. The cere- 

 bellum is always very narrow (in the antero-posterior direction), 

 but especially so in the early stages of its formation, when it is 

 very minute. It is larger in the adult than in any larval stage. 



2. The Spinal Cord. — The spinal cord of the lamprey is, 

 as has long been known, remarkable for its curiously-depressed 

 shape, which, in tranverse section, appears like that of a very 

 much flattened bean. This peculiarity is not known to occur 

 outside the group Cyclostomata, and it is, therefore, interesting 

 to observe that the character is not an original one, strongly 

 marked from the beginning, but is, on the other hand, of com- 

 paratively late appearance. In advanced embryos, the spinal 

 cord has a greater vertical than transverse diameter ; the lumen 

 is contracted in the middle and somewhat expanded dorsally 

 and ventrally, having thus a section somewhat like the figure 8 ; 

 the fibres appear as two isolated bands along the sides (Fig. 38, 

 PI. XI). The roof and floor of the canal aire, as in the case of 

 the brain, formed by a single layer of cells. Shortly after hatch- 

 ing the fibrous bands increase very much, and extend around 

 the entire circumference of the cord, which, however, still retains 

 its former shape. In Ammocoetes of 7 mm. length the change 

 of form begins to appear (Fig. 44, PI. XI) ; the transverse 

 diameter of the cord is now greater than the vertical, the bands 

 of fibres being especially thick along the sides ; the ganglion 

 cells are reduced in number, and have commenced to extend 

 laterally; the canal is much smaller, widening below, and 

 reduced to a mere slit above ; a slight protuberance or keel 

 runs along the ventral median line of the cord. In the next 

 stage, of 14 mm. (Fig. 45), these changes are carried still 



