On the Relation of the Metamerie Segmentation 

 of Mesoblast in Petromyzon to that in 

 Amphioxus and the Higher Craniota. 



{Preliminary) 

 BY 



S. Hatla. 



It has been very well known for a long time that the mesoblast 

 in all Vertebrata undergoes the metamerie segmentation only in its 

 dorsal part adjacent to the chorda and neural canal, while the ventral 

 part remains unsegmented. Further, efforts have been made to bring 

 the fact in harmony with the corresponding process observable in Am- 

 phioxus. Setting aside innumerable literature on the subject, I will cite 

 here O. Hkrtwig's 1j formulation of the fact, which runs as follows : 

 " Ahnliche Vorgange (as in Amphioxus) vollziehen sich in etwas abgeänd- 

 erter Weise bei den übrigen Wirbeltbieren " (p. 139). How far the mode 

 is modified, we see in the following lines given by the same author 2 ': 

 " Während nun beim Amphioxus der Process der Segmentirung sich 

 auf das gesatumte mittlere Keimblatt ausgedehnt, ergreift er bei den 

 Amphibien und ebenso bei allen übrigen Wirbeltbieren nur den Theil, 

 welcher an Chorda und Nervenrhor angrenzt, lässt dagegen die Seiten- 

 platten unberührt " (p. 140). It is easily conceivable that there is a gap 

 between the case in the Craniota and that in Amphioxus, although there 

 would be no difficulty in accepting that the process in both cases is 

 analogous, and the parts thus formed are homologous with each other. 

 But the attempt to derive one type from the other is, so far I am aware, 

 has not been supported with any positive evidence for it 



1) Lehrbuch der Entwicklungsgeschichte der Menschen und der Wirbelthiere . 

 3. Auflnge, 1890. 

 '2.) Loc. cit. 



