112 Journal of Travel and Natural History 



SYMMETRY AND HOMOLOGY IN LIMBS.* 



■pvR. JEFFRIES WYMAN is publishing, in the " Proceedings " 

 -'-^ of the Boston Society of Natural History, the results of 

 many years' studies upon the Symmetries of the Body and the 

 Homologies of the Limbs in Vertebrates. "The fundamental idea 

 of the organs of organic life," he says, " involves the condition of 

 symmetry " — a symmetry which, however distorted, is apparent on 

 either side of every plane (not diagonal) dividing the body into 

 equal halves. At the outset he urgently insists upon the necessity 

 of examining this subject by the light which embryology affords ; 

 this will furnish ample proof of the complete symmetry of plan in 

 the extremes of vertebrates. 



In the early stages of development, symmetry is maintained at 

 the two ends as well as at the two sides of the body ; but, subse- 

 quently, it is more or less modified as the animal is adapted to its 

 special conditions of life. This symmetry of fore and hind parts 

 — a most important guide in determining homologies — results quite 

 naturally from the mode of growth of an embryo and of its organs. 

 The growth of the embryo is from a central point forward and 

 backward ; the nervous axis, in its earliest stage, is a groove which 

 is symmetrically enlarged at either end, and gradually closes from 

 the centre outward ; the lung and allantois — in air-breathing 

 animals — arise as simple sacs at opposite ends of the straight sym- 

 metrical intestine. But it is in the limbs that the most striking 

 feature of this kind of symmetry can be traced. Placing the body 

 of an embryo in an horizontal position, the limb buds in their 

 earliest stage are simply tegumentary outgrowths, projecting at 

 right angles from the body ; when these limbs subsequently bend 

 at a point near the trunk, their ends are directed downward ; the 

 hands and feet become vertical, facing inward ; and the upper arms 

 and thighs project at right angles from the body. At a later period, 

 the elbow swings backward and the knee forward against the body, 

 while the fore-arm slowly rotates inward, until the palm, like the 

 foot, faces almost backward. 



Bearing these embryonic changes in mind, the homotypes in 



* "On Symmetry and Homology in Limbs." By JcfTrics Wyman, M. D. 

 8vo. pamphlet, 45 pages. Boston, 1868. 



