64 COMPAEATIVE ANATOMY. 



§ 54. 



I. General Homology is under consideration "when an organ 

 is referred to a category of organs, or wlien a single organ com- 

 pared with another is taken merely as the representative of such a 

 category. These categories always consist of several organs or 

 parts present in the body. When we compare the body-segments 

 of an Annelid, the vertebra, or the appendages of an animal with 

 one another, we lay the foundations of general homology. This 

 again consists of several subdivisions, according to the kind of 

 category which is made use of in the comparison. 



1) Homotypy has reference to organs which are fellows to one 

 another, such as the organs of the two sides of the body ; the right 

 kidney is homotypical with the left, and the right eye with the left 

 eye, and so on. Whilst these examples may not show the necessity 

 for the formation of this division, it should be noticed in addition 

 that homotypical organs have not always the same characters. They 

 are often so changed that their homotypy cannot be recognised, 

 and has to be worked out. 



2) Homodynamy (equivalent to the general homology of Owen, 

 and partly also to his serial homology) subsists between parts of 

 the body which are affected by a general morphological pheeno- 

 menon serially expressed in the organism. Homodynamy is dis- 

 tinguished from the next subdivision by the fact that the parts 

 in question are arranged along the long axis of the organism and 

 define its type. The metameres therefore are homodynamous parts ; 

 as are the segments of the Arthropoda, the primitive vertebrae of 

 the Vertebrates, etc. 



3) Homonomy. This describes the relation to one another 

 of those parts which are arranged along a transverse axis of the 

 body, or in one segment only of its long axis. The rays of the 

 pectoral and pelvic fins of fishes, the individual fingers and toes 

 of the higher Vertebrata are homonomous structures. 



Besides these there are other subdivisions of general homology 

 distinguishable, which are however of very subordinate importance. 



§ 55. 



11. Special Homology, Homology in the restricted 

 sense. This is the name we give to the relations which obtain 

 between two organs which have had a common origin, and which 

 accordingly have also a common embryonic history. As exact 

 proofs of genetic relations are necessary for the investigation of 

 special homologies, this mode of comparison is generally limited 

 in the lower divisions of the Animal Kingdom to systems of organs ; 

 it is only in the Vertebrata that it is possible to extend this method 

 to more minute features. Thus among the Vermes or the Mollusca 

 we can hardly indicate, with any certainty, particular parts of 



