256 SCOTT. - [Vol. I. 



but in relation to the size of the animal, even the hind-brain is 

 seen to be exceedingly small, very much smaller than in the 

 Selachii or Amphibia, for example. 



In the embryo of Pctromyzon nearly ready for hatching, or 

 in the newly hatched larva, the cranial flexure is comparatively 

 slight. At its maximum it scarcely, if at all, exceeds a right 

 angle, and, as we have already seen, the flexure is gradually 

 corrected. In the higher vertebrates, the correction of the 

 cranial flexure is, as Balfour has shown, only apparent, and is 

 brought about by foldings in the opposite direction, the effect 

 of which is to apparently straighten the axis of the brain, but 

 in reality to make it more sinuous than before. Something of 

 this may also be seen in Pctromyzon^ where the dorsal fold 

 which marks the limits of the mid and hind-brains becomes 

 gradually deepened as the correction proceeds, and the cerebral 

 hemispheres are folded slightly upwards and backwards. But 

 these processes have little effect, and in addition to them there 

 is a real correction of the flexure brought about by an upward 

 rotation of the fore and mid-brains about a transverse axis. 

 The correction, though real, is not altogether complete ; as may 

 be seen in Fig. 13, PI. IX, there is still a slight bend in the 

 axis of the brain. 



(«) The Fore-Brain. — At the time of hatching the fore- 

 brain consists of the thalamencephalon and the hemispheres, 

 the olfactory lobes not having yet become differentiated 

 from the latter. The hemispheres, as I showed in my former 

 paper, arise as an unpaired and solid rudiment, which later 

 divides into two portions; these remain solid for a considerable 

 period, nor have I been able to observe any trace of the lateral 

 ventricles earlier than in larvae of 14 mm. in length. In 

 young larvae the hemispheres are very minute, and they enlarge 

 but slowly, as their small size is characteristic of full-grown 

 Ammocoetes, and even of the adult. The olfactory lobes are not 

 formed, as Shipley (37) has supposed possible, from special 

 ganglionic detachments of the olfactory and pituitary epi- 

 thelium ; but by enlargement and constriction of the anterior 

 portion of the hemispheres. Shortly after their formation a 

 small lumen appears in each one, which communicates by a 

 common chamber with the lateral ventricles of the hemispheres 

 (Fig. 16, PI. IX). At first the lobes are smaller than the hem- 



