THE DEVELOPMENT OF FASCIOLAKIA. 



H7 



" crowd in at the neck of the constriction " do'more than this, for 

 some of them coming from opposite sides join to form a plug 

 which projects into the hollow of the rounded excretory organ. 

 (Fig. 4, />/.) As the excretory organs at later stages have no 

 lumens whatever, I conclude that these cells become modified 

 secondarily into excretory cells like the primary ones. 



The second important change which takes place in the external 

 kidneys involves their position and results in the adoption of that 

 curious relation with the velum which led Osborn ('85) to speak 



ExK 



ExR 



Fi<;. 5. Four stages in the migration of the external kidneys (ex.k. ) from the 

 lateral surface of the cannibal to their final subvelar position on the veliger. 



of them as " subvelar masses." When this change beeins the 



o o 



embryo as a whole has enlarged greatly (Fig. 5), and has both 

 by growth and by the stretching, due to the ingestion of eggs, 

 become much wider than it was, especially at the base of the 

 head vesicle. This increase in size brings about the removal of 

 the external kidneys from the anterior suiface of the larva to the 

 lateral surfaces. Later the great activity in the velar ridge, by 

 which this becomes prominent, as well as the growth of the whole 

 anterior end of the larva, result in lifting the external kidneys 

 upward and carrying them laterally away from the central mass 

 of yolk on which at first they lie directly. Further lateral growth 



