1 40 Embryology and Growth of Fishes 



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

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

 lar\'al stage is actually initiated: thus in sharks the excessive 

 amount of yolk material which has been provided for the growth 



Fio. 101. — Larva of the Sail-fish, htiophor^is. 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 f onn 

 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. Angnilla chrisijpa Rafinesque. Family AngidUidce. 



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

 and the following structures, characteristic in outward appear- 



