40 



PETER OKKELBERG 



occur at first only in the posterior region of the body, but they 

 gain a more cranial position with the growth and development of 

 the larvae. The movement of the germ cells craniad is probably 



a passive one in early stages and results from a shifting of the 

 tissues during development. In later stages, after the cells 

 have lost their yolk, migration is probably due partly to an active 



