AVyman.] 254 [June 5, 



The statements which have been made in the previous pages are 

 intended to show that even in right and left parts, symmetry is of vari- 

 ous degrees, and is rarely, if ever, absolute. Asymmetry, 1 owever, 

 is in most cases slight, but may in certain others become as great as 

 that between fore and hind limbs, in accordance with the degree of 

 difference in the function of the two sides. These differences do not, 

 nevertheless, prevent our recognizing the idea of symmetry of plan 

 underlying the structure of such parts. In comparing fore f nd hind 

 limbs, it has been shown that they are sometimes nearly symiaetrical, 

 but generally the symmetry is largely distorted. But if we bear in 

 mind the fact that the limbs which in the adult are the most uiisym- 

 metrical, are quite symmetrical in the embryo, tlie hypothesis that the 

 idea of symmetry underlies their structure is rendered highly prob- 

 able. 



Analogy between symmetry and polarity. From what has beeti stated 

 it is obvious that in the early stages of development there is at work 

 a force which regulates the distribution of the particles of matter out 

 of which the embryo is formed in a symmetrical manner, and that up 

 to a certain stage there is symmetry, not only of right and left, but 

 of fore and hind parts. The essential characteristic of this force is 

 that it gives rise to similar but reversed forms on the two sides as well 

 as on the two ends of the axis of the body. 



If we look for any thing among known forces analogous to this 

 force, it is to be found, if anywhere, in those known as polar forces. 

 The essential features of polarity, as of symmetry, are antagonism, 

 oppositeness or inversion, either of qualities or forms. Studying the 

 subject from the most general point of view, there are striking resem- 

 blances between the distribution of matter capable of assuming a polar 

 condition, and free to move around a magnet, and the distribution of 

 matter around the nervous axis of an embryo. 



In every complete series of magnetic curves formed by particles in a 

 polar condition, there are two neutral lines (Fig. 6, A), one extending 

 len'Tthwise of the magnet, so that the curves formed may be divided 

 into right and left ; secondly a transverse one, the particles on each 

 side of which form the north and south curves, or which for purposes 

 of comparison might be called fore and hind curves. In the right and 

 left series those which are on one side of the long a.xis are symmetrical 

 Avith those on the other and not in themselves, and in the north and 

 south series those on either side of the transverse neutral line ai'e sym- 

 metrical witli each other, and not in themselves. 



If these curves are projected on paper, and this be folded on the 

 line of the longitudinal, or north and south axis, the curves of opposite 

 sides or opposite ends will correspond as right and left hands or other 

 double organs do when applied to, or placed opposite each other. 



