Chapter 27 



HOMOLOGY; ANALOGY; AUTOTOMY; 

 REGENERATION; MORPHOGENESIS 



HOMOLOGY (ho-moroji) (Gr. homos, same; logos, discourse) 



Homologous organs or structures are those which are jundamentally 

 sim.ilar in structure and in embryologic developm,ent, having their origin 

 in a common ancestral type. The arms of man, the forelegs of cats, the 

 wings of birds, etc., are structurally homologous and morphologically 

 similar in spite of their apparent differences upon casual observation. If 

 they are compared carefully and in detail, they will be observed to be 

 quite alike. In this case, two homologous bones, one small and the 

 other large, may make the over-all picture of the two appendages ap- 

 pear to be more different than they are fundamentally. 



Several appendages on the same organism may show homology be- 

 cause of their similar embryologic origin and development as well as 

 their similar structure in the adult. For example, the pairs of appen- 

 dages on a crayfish or lobster (Figs. 128-130, 307) show homology, and 

 since the appendages form a consecutive series, it may be called serial 

 homology. The apparently different appendages of the crayfish have 

 evidently developed from a fundamental type and in the adult are con- 

 structed along fundamentally similar lines, even though some of them 

 perform different functions. 



Likewise, the legs of different insects are structurally homologous (Fig. 

 308) since they are composed of fundamentally the same units, some of 

 which may vary in size or shape, which makes the legs of different insects 

 appear more different than they actually are. 



Homologous structures or organs may have similar functions (for 

 example, the legs of man and the hindlegs of cats, dogs, horses, etc.) or 

 they may have different functions (for example, the arms of man and 

 the wings of birds). A study of homology teaches that certain struc- 

 tures may be more closely related embryologically and structurally than 

 a casual observation of them might reveal. 



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