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been expressed in the phrase "ontogeny recapitulates phylogeny," 

 ontogeny being defined as the development of the individual, completed 

 within a single lifetime, and phylogeny as the development of the race, 

 covering, perhaps, ages of time and unnumbered generations. The 

 biogenetic law has also been considered as illustrated by a correspondence 

 between the egg cell and a single-celled animal; the blastula and a colonial 

 protozoan; the gastrula and a supposed gastrula-like ancestor of the 

 Metazoa called a gastraea; and the triploblastic embryo on the one hand 

 and a triploblastic animal on the other. 



This conception has been a fruitful one in its influence upon zoological 

 progress, since it has directed attention to the broader principles under- 

 lying embryological development. However, it has also been criticized 

 very severely because its proponents have applied it in too sweeping a 

 manner and without due consideration of the fact that the animal king- 

 dom represents many lines of descent and that resemblances may be the 

 result not of common ancestry but of similar adaptations arrived at 

 independently and adjusting unrelated forms to similar environments. 

 It is interesting to note, in this connection, that while the shrimp possesses 

 all those free-living larval types and thus shows a complicated meta- 

 morphosis, the crayfish, in the same group, which carries its young 

 about on its swimmerets, shows no metamorphosis at all. 



