140 Embryology and Growth of Fishes 



a larva as soon as it emerges from its egg-membranes. In some 

 instances, however, it is difficult to decide at what point the 

 larval stage is actually initiated: thus in sharks the excessive 

 amount of yolk material which has been provided for the growth 



FIG. 101. Larva of the Sail-fish, Istiophonis, very young. (After Liitken.) 



of the larva renders unnecessary the emerging from the egg at 

 an early stage; and the larval period is accordingly to be 

 traced back to stages that are still enclosed in the egg-mem- 

 branes. In all cases the larval life may be said to begin when 



FIG. 102. Larva of Brook Lamprey, Lampetra wilderi, before transformation, 

 being as large as the adult, toothless, and more distinctly segmented. 



the following conditions have been fulfilled: the outward form 

 of the larva must be well defined, separating it from the mass of 

 yolk, its motions must be active, it must possess a continuous 

 vertical fin-fold passing dorsally from the head region to the 



FIG. 103. Common Eel. Anguitta chrisypa Rafinesque. Family Anguitlidce. 



body terminal, and thence ventrally as far as the yolk region; 

 and the following structures, characteristic in outward appear- 



