254 SCOTT. - [Vol. I. 



opportunity of investigating it, that I am as yet not convinced 

 of the accuracy of Shipley's views. 



I am under great obligations for assistance and supplies of 

 needed material to many persons. First and foremost to 

 Professor Gegenbaur, who placed Calberla's collections at my 

 disposal, and who constantly exhibited the greatest interest and 

 kindness while I was in his laboratory. Professors Balfour, 

 Benecke, Wiedersheim, and Gage have sent me material of 

 much value, and Mr. Shipley has most kindly sent me an ex- 

 tensive series of his beautiful preparations, which have proved 

 of the utmost service. To all these gentlemen I wish here to 

 express my very cordial thanks. 



I. The Central Nervous System. 



In order to understand the alterations in the brain and in 

 the position of the sense-organs, it will be necessary to take 

 into account the changes which take place in the character 

 and shape of the mouth, as they have a profound effect upon 

 the position of the surrounding organs. The mouth in an 

 embryo of seventeen days (Fig. i, PI. VIII) is a deep depres- 

 sion of the epiblast, very large internally, but with a somewhat 

 contracted opening. The upper lip is short, and, seen in 

 longitudinal section, has a rounded anterior margin, and does 

 not descend to the level of the lower lip. By the eighteenth day 

 (Fig. 2) the cranial flexure has increased, and, indeed, attained 

 its maximum ; the nasal pit opens directly downwards ; the 

 upper lip has become longer, and now extends somewhat below 

 the level of the lower lip ; it has also extended antero-pos- 

 teriorly, deepening the nasal involution, and becoming some- 

 what triangular in sagittal section. At this stage the mouth is 

 altogether ventral in position, and when the head of the entire 

 embryo is viewed from below, the resemblance of the mouth to 

 that of a Selachian in shape and position is very striking. 

 Nothing could be more different from the suctorial disk of the 

 adult lamprey, or even from the mouth of the larva, than the 

 shark-like mouth of the embryo at this stage. Shortly after 

 hatching (Fig. 3) a remarkable change takes place: the cranial 

 flexure begins to correct itself by an upward rotation of the 

 fore and mid-brain, about a tranverse axis passing through the 



