LARVAL FISLLES 21 3 



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, Ay 

 and mouth, M, are making their appearance. 



D. THE LARVAL DEVELOPMENT OF FISHES 



When the young fish has freed itself from its Qgg 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 Qgg 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 Qgg at an early stage ; and the larval 

 period is accordingly to be traced back to stages that are 

 still enclosed in the Qgg membranes. In all cases the 



