Homologue

(hom'-o-Iog). An homologous organ common to any number of species, cla es, or orders of animals; that which has the same relative structure, position, or development as another. The same organ in different organisms under every variety of form and function. We may have: I. Homogenetic Homologues, i.e., parts which, on the evolutionary theory, have a genetic relation, e. g., the humerus of a horse and that of an ox. 2. Homoplastic Homologues, i. e., parts closely similar as to relative position, but with no genetic affinity, or only a remote one, e. g., the ventricle of a bat and that of a bird. 3. Homogenetic Serial Homologues, i.e., different parts of one individual, the correspondence between which is to be accounted for by the influence of external conditions, e.g., the fore-limbs and hind-limbs of one side of any animal. 4. Vertical Homologues, i. different corresponding parts of one individual, of which one part is dorsal, the other ventral, e.g., dorsal and anal fins of certain fishes. 5. Lateral Homologues, i. e., different corresponding parts of one individual, of which one part is right and the other left, e. g., the right and left arms of a man. 6. Antero-posterior Homologues, i. e., different corresponding parts of one individual, of which one part is anterior, the other posterior, e.g., the mouth and the cloaca. 7. Radial Homologues, i. e., different corresponding parts of one individual which radiate from a central axis, e. g., any two arms on the same starfish. 8. Homo trophic Homologues, i. e., parts of an individual, the correspondences between which are regarded as due to internal modifying influences, e.g., the eyes and the ears. 9. Act i no logo us Homologues, i. e., succe ive subdivisions, parts, or regions of a part or segmented organ, e.g., two succe ive joints of an antenna of a lobster, or the vertebral and sternal parts of the same costal element. 10. Special Homologues, i. e., parts of two organisms belonging to the same special skeletal category, e.g.. the bas i-occipital of man and the cod. II. General Homologues, i. e., parts of two organisms belonging to the same general skeletal category, e.g., the vertebral center of man and that of the cod, or the lateral appendages of the lobster and those of the scorpion. (Mivart.) Homology (ho-mol' -o-je).