Wyraaii.] 258 [June 5, 



"caudate appendages," or processes which connect them with the nerve 

 tubes. 



Owen appears to make use of the term polarity in the sense in 

 which it is made to stand as representing the quality of a force acting 

 in animal bodies, and producing symmetrical results. After compar- 

 ing the dorsal and abdominal portions of the vertebral arches, and 

 showing that they repeat each other, he says "symmetry and polarity, 

 or serial homology of the parts of the same vertebral segment is usually 

 still more strictly observed in the transverse direction, and is so obvi- 

 ous as to have immediately led to the detection of the homologous 

 parts, which are accordingly distinguished as right and left."* He 

 does not, however, recognize the symmetry of fore and hind parts. 



Prof Dana uses the term in the same manner. "An animal is em- 

 bodied or concentrated force, which force manifests polarity in the 

 results of its action in development, that is in the oppositeness of the 

 anterior and posterior extremities of the structures evolved, and also in 

 the dorso-ventral relation of these structures." f 



Polarity, accoi-ding to Mr. Faraday, may be considered as "an axis 

 of power, having contrary forces exactly equal in opposite directions." 

 This power will produce perfectly symmetrical figures, however, only 

 Avhen wholly free from the influence of a superior force. In the pres- 

 ence of such it may be interfered with, or have the results of its ac- 

 tion so changed as to give rise to forms whose symmetry is more or 

 less distorted, as is so frequently the case in crystals; these, we are 

 told, being exactly symmetrical only in idea. 



In the vertebrate animal, the plan of construction appears to be in 

 accordance with the idea of general symmetry, which in the early 

 stages of development is maintained at the two ends as well as the 

 two sides of the axis, but subsequently is more or less interfered with to 

 adapt the animal to its special conditions of life. This force, produc- 

 ing symmetry, acts in a manner analogous to a polar force. To desig- 

 nate the disturbing, or rather adapting force, by which the force 

 tending to act in a symmetrical manner is interfered with, we must 

 still retain the term vital or life-force, although this may in the end 

 prove only a physical force acting under special conditions. 



Homology. Owen defines a homologue to be "the same organ in 

 different animals under every variety of form and function." % When 

 parts are repeated in the same animal, not from right to left, but from 

 before backwards, either on the middle line of the body, as the ver- 

 tebra? or sternal pieces, or on the same side, as the ribs, such parts 

 are homologous, but not in the same sense as when they are re- 



* Archetype and Homologies of the Vertebrate Skeleton. London, 1848. p. 166. 

 t Am. Journal of Science. Vol. xxvii, p. 157, 18u4. See also Vol. lxi, p. 154, 

 1866. See also Wilder op. cit. p. 9. 

 t Archetype and Homologies of Vertebrate Skeletons, p. T. 



