H4 Journal of Travel and Natural History 



only two phalanges occur in these organs, whatever their variety in 

 other respects ; on the other, the fact, that when the limbs are 

 placed in truly symmetrical positions, the great toe is found on the 

 inside, the thumb on the outside of the limb. Other minor con- 

 siderations are discussed, shewing that our author inclines to view 

 the outer toe as the homotype of the thumb. 



It is not, however, to these special parts alone that the principle 

 of reversed symmetry may be applied ; the scapular and pelvic 

 arches may be compared, and the scapula shewn to be homologous 

 with the ilium, the clavicle with the ischium, and the coracoid with 

 the pubes. Believing that the scapular arch cannot be considered 

 an appendage to either the cranial, cervical, or thoracic vertebrae, 

 Dr Wyman objects to the view of Owen and those of his prede- 

 cessors who have regarded both of these arches as ribs, and suggests 

 that we need to seek further evidence from embryology before 

 drawing any definite conclusions. From his own observations he 

 has furnished only negative evidence, shewing that the develop- 

 ment of the pelvis in frogs is dissimilar to that of the ribs. 



Nor does he agree with most authors in considering the limbs 

 appendages of the vertebral column. Struck by the fact, that in 

 the embryo they bear so strong a resemblance to the median fins 

 of fishes or the flukes of cetaceans, he thinks " there is ground for 

 the hypothesis, that limbs belong to the category of tegumentary 

 organs, and that their connexion with the vertebral column through 

 the scapular and pelvic arches is only secondary." Like the 

 adipose fins of fishes, they are outgrowths of the tegumentary cells, 

 which, for a time, undergo no diff"erentiation into bones or other 

 tissues ; they are not forced outward by the protrusion of the bones, 

 but the bones are afterwards developed in the limbs and grow 

 pari passu with them. Moreover, the limbs have no connexion 

 at first with the scapular and pelvic arches, but become at a later 

 period attached to them as teeth become connected with the jaws. 



In his discussion of the different kinds of symmetry, and 

 especially of that maintained at opposite ends of the body. Pro- 

 fessor Wyman has drawn a curious comparison between symmetry 

 and polarity, between the distribution of matter around the nervous 

 axis of an embryo, and the distribution of polarized matter around 

 a magnet. Just as in the latter case there are two neutral lines, 

 one extending along the middle of the magnet, and the other at 

 right angles to it — the particles divided by the second line forming 

 the north and south cur\'es — so we have a symmetrical arrange- 



