LARVAL FISHES 213 
invaginated tract of ectoderm, EC, fuses, and at this point 
an opening is later established. 
In Fig. 283 has been figured a late embryo. This may 
be compared with that of the sturgeon of Fig. 264. The 
Teleost, though of rounded form, is the more deeply im- 
_ planted in the yolk sac; it is transparent, allowing noto- 
chord, primitive segments, heart, and sense organs to be 
readily distinguished; at about this stage both anus, A, 
and mouth, J/, are making their appearance. 
D. THE LARVAL DEVELOPMENT OF FISHES 
When the young fish has freed itself from its egg mem- 
branes, it gives but little suggestion of its adult form. It 
enters upon a larval existence; which continues until matu- 
rity. The period of metamorphosis varies widely in the 
different groups of fishes—from a few weeks’ to longer 
than a year’s duration ; and the extent of the changes that 
the larva undergoes are often surprisingly broad, invest- 
ing every organ and tissue of the body,—the immature 
fish passing through a series of form stages which differ 
one from the other in a way strongly contrasting with the 
mode of growth of amniotes; since the chick, reptile, or 
mammal emerges from its embryonic membranes in nearly 
its adult form. 
The fish may, in general, be said to begin its existence 
as 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 pro- 
vided for the growth 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 membranes. In all cases the 
