278 bulletin: museum of compaeative zoology. 



This investigation has been made iu the Zoological Laboratory of the 

 Museum of Comparative Zoology at Harvard College. I gratefully ac- 

 knowledge the valuable assistance and advice of its Director, Prof. E. L. 

 Mark, at whose suggestion the woi'k was undertaken. I am indebted to 

 Alexander Agassiz for the privilege of studying at his private labo- 

 ratory in Newport ; also to Professor Mark for embryonic material of 

 Petromyzon, and to Miss Julia B. Piatt for embryonic material of 

 Amphioxus. 



