VIII 

 THE DEVELOPMENT OF FISHES 



The groups of fishes have hitherto been contrasted 

 in the structures of their living and fossil forms. They 

 should next be reviewed in the light of their mode of 

 development ; for the developmental stages of the Shark, 

 Lung-fish, or Teleostome might be expected, according 

 to time-honoured belief, to furnish important evidence 

 as to their descent and interrelationships. The younger 

 stages of the various forms of fishes should thus suggest 

 their ancestral characters : the developing Teleost should 

 approach the Ganoid ; the Lung-fish and the Ganoid 

 should resemble their supposed elasmobranchian ancestor. 



But the embryology of fishes is in this regard very 

 inconclusive, if at present in any important way sugges- 

 tive. The majority of the forms, including some of the 

 most important, are developmentally unknown ; yet suffi- 

 cient is known of the representative members of the 

 groups to show the most perplexing characters. On the 

 one hand, the developmental processes of forms which are 

 regarded by the morphologist as closely akin seem often 

 widely distinct ; and, on the other hand, the fishes which 

 should, a priori, exhibit an archaic mode of development 

 actually present complex processes of early growth which 

 can only be interpreted as highly specialized. In fact, 



there are far greater differences in the developmental plans 



179 



