278 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY, 
This investigation has been made in the Zoölogical Laboratory of the 
Museum of Comparative Zoölogy at Harvard College. I gratefully ac- 
knowledge the valuable assistance and advice of its Director, Prof. E. L. 
Mark, at whose suggestion the work 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. Platt for embryonic material of 
Amphioxus, 
