1867.] 257 [Wyman. 



In comparing the results of the symmetrically acting force in ani- 

 mals with a polar force like that of magnetism, it is not intended to 

 imply that the two forces are the same, but only that they have like 

 modes of acting, and that when left to themselves undisturbed bj^ 

 other influences, each tends to produce symmetrical figures. The 

 type or general idea of any of the double monsters may be imitated 

 by the combined action of two magnets. 



In the preceding paragraphs we ' have S23oken of the symmetrical 

 di.<<tribution of the particles of matter around the nervous axis as if 

 the distributing force emanated from this axis. It is not to be inferred 

 that such is actually the case, since the force is already in action be- 

 fore the nervous axis itself is formed, and may be said to be manifested 

 in the first stage of the segmentation of the yelk ; for when the whole 

 yelk, or as in some cases, a limited portion of it, divides into two dis- 

 tinct segments, such division shows that a symmetrically acting force is 

 already present ; in fact, we have now right and left parts. Such force 

 is again manifest when a new division takes place at right angles to 

 the first, separating each right and left portion into two others, which 

 may be compared to the fore and hind divisions of the body ; even the 

 nervous axis itself is symmetrically developed under the influence of 

 this same force. The nervous axis, however, when formed, coincides 

 with the axis of the symmetrically-distributing force. 



The use of the term polai-ity in connection with organic structures 

 has long been familiar to physiologists, but apparently with very varied 

 signification. Oken in his celebrated "Programm" uses the following 

 words with regard to the skeleton : "This skeleton repeats itself at the 

 two poles ; each pole repeats itself in the other, and they are head 

 and pelvis." It does not appear from this, nor from any other state- 

 ment of his that we have seen, precisel\' what he understood by the 

 Avord pole. At the present day few will agree with him that the head 

 and pelvis repeat each other. Still, although these parts are not com- 

 parable, the idea underlying his statement, viz., that the two ends of 

 the body do repeat each other, may be, and we believe is, correct. 



In order, however, that they may be repeated as if under the influ- 

 ence of a polar force, or of a force acting in a manner analogous to 

 one, there should be a more or less symmetrical repetition of homolo- 

 gous parts. 



The term pole is often used in the description of eggs and of cells ; 

 in the former to distinguish the portion of the egg where the oily mat- 

 ters are collected from the opposite side where there is only albumen, 

 and in the latter, simply to designate the two ends without intending 

 thereby to imply any difference of quality or force. In the nervous 

 system cells are described as "unipolar," "bipolar," or "multipolar," 

 which only means that they are .prolonged into one or more jioints, 



PEOCEEDINGS B. S. N. H. — VOL. XI. 17 DECEMBER, 1867. 



