Embryology and Growth of Fishes 139 



ectoderm fuses, and at this point an opening is later estab- 

 lished. 



The late embryo of the Teleost, though of rounded form, 

 is the more deeply implanted in the yolk-sac than that of the 

 sturgeon; it is transparent, allowing notochord, primitive seg- 

 ments, heart, and sense-organs to be readily distinguished; at 

 about this stage both anus and mouth are making their appear- 

 ance." 



The Larval Development of Fishes.* "When the young 

 fish has freed itself from its egg-membranes it gives but little 



' 



FIG. 99. Young Sword-fish, Xiphias gladius (Linnaeus). (After Liitken.) 



suggestion of its adult form. It enters upon a larval ex- 

 istence, which continues until maturity. The period of change 

 of form varies widely in the different groups of fishes, from 

 a few weeks' to longer than a year's duration ; and the extent 



FIG. 100. Sword-fish, Xiphias gladiiis (Linnaeus). (After Day.) 



of the changes that the larva undergoes are often surprisingly 

 broad, investing every organ and tissue of the body, the imma- 

 ture 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 



*This paragraph is condensed from Dean's "Fishes Living and Fossil " 



