No. 2.] DEVELOPMENT OF PETROMYZON: 255 



mid-brain, so that the lamina terjninalis which formerly 

 faced directly downwards, now faces obliquely forwards and 

 somewhat downwards (Fig. 3). The upper lip has greatly 

 increased in size, especially in the vertical direction, the 

 posterior edge growing out into a long process. This growth 

 keeps pace with the correction of the cranial flexure, so that 

 the upper lip still extends down to the level of the lower, and 

 the mouth is still ventral in position. Later, however, the 

 upper lip itself begins to rotate about a transverse axis ; the 

 point marked ULin the drawings, which in Figs. 2 and 3 points 

 backwards, in Fig. 12 (Ammocoetes of 12 mm.) points directly 

 forwards, having thus moved through an arc of 180^, At the 

 same time the lip increases in vertical thickness, and thus com- 

 pletely encloses the olfactory epithelium, which was at first 

 exposed on the surface, but now comes to lie deep within the 

 head. Further, this rotation and method of growth of the 

 upper lip transfers the opening of the nasal pit from the ventral 

 to the dorsal surface of the head. Several intermediate stages 

 of this transformation between those shown in Figs. 3 and 12 

 might be given, but they are doubtless sufficient. 



I. The Brain. — With the exception of the olfactory lobes, 

 the brain of the larva at the time of hatching contains all the 

 parts which are to be observed in the adult brain, though the 

 structure and proportions of the various parts are very different 

 from those found in the later stages of larvae, and still more so 

 from those of the adult. The brain, as a whole, is exceedingly 

 small compared with that of fishes or Amphibia at a corre- 

 sponding stage of general development, and this reduction is 

 especially marked in the fore and mid-brains, which may fairly 

 be called minute. The exceedingly small size of these parts is 

 no doubt due to the retarded development of the sense-organs 

 connected with them, as both the olfactory organ and the eye, 

 especially the latter, remain in a very backward and incomplete 

 state during the whole of larval life. The larva lives, as is well 

 known, buried in sand or mud, and the organs of higher sense 

 are nearly functionless ; this condition must necessarily have a 

 very considerable effect upon the growth of the brain. Wieder- 

 sheim (41) has called attention to the very large size of the 

 hind-brain in the Ammocoete. This is quite true of the hind- 

 brain, as compared with the other divisions of the encephalon, 



