1887.] NATURAL SCIENCES OF PHILADELPHIA. 361 



limbs, because it can be shown that what is metapterygiura in one 

 case is not such in another; besides, there are embryological difficul- 

 ties in the way which are insuperable. The exact homological 

 equivalency of pro- meso- and metapterygium, has in fact, been aban- 

 doned by some of the ablest contemporary anatomists. 

 4. Formation of plexuses and their inexact homology. 



There is no more reason to supjDOse that the nerve plexuses of 

 vertebrates are exactly homologous, than there is for the supposition 

 that the muscles are exactly homologous. The trapezius and latis- 

 simus dorsi for example, cannot be regarded as having exactly the 

 same morphological value in Man, Selachians and Amphibians, be- 

 cause in these three cases they are not derived from the same num- 

 ber of somites; they are only physiologically homologous. 



The arguments of Gegenbaur, Fiirbringer and DavidofF that the 

 limbs have migrated backwards or forwards, as indicated by the ex- 

 istence of the collector nerves formed by the ansse and commissures 

 between successive pairs, anterior and posterior to those pairs which 

 form the functional plexus of the limb, are not sustained by em- 

 bryological evidence, and the existence of the nervus collector is 

 rather to be taken as evidence that the radii belonging to the pairs 

 entering into the anterior and posterior portion of the n. collector 

 have been suppressed or fused with the radii forming the peduncle 

 of the limb. My reason for holding this opinion is, that the only 

 case in which the effect of translocation of a limb on the peripheral 

 ends of the nerve pairs passing to that limb, has been traced embry- 

 ologically, shows that their peripheral ends travel with the displaced 

 limb, at the same time retaining their origins, and do not run par- 

 allel for a long distance with the functional pairs, as is shown by 

 Davidoff's own figures of the nerve plexus of Acanthias. 



Whatever fibres of the collector nerve enter into the plexus of the 

 functional limb, have been incorporated in vii'tue of the constriction 

 of the limb fold posteriorly and anteriorly, as a result of which many 

 radii which were originally attached to the sides of the body, have 

 acquired a secondary attachment to the proximal ends of the blended 

 radii, from which the so-called pro- meso- and metapterygium have 

 been evolved. There can be no doubt of the fact, that in this way 

 the limbs of primitive vertebrates first became pedunculate. It can 

 thus be shown that the radii which are detached from the body, are 

 not lost but simply carried farther out by the accelerated growth of 

 the radii forming the skeleton of the peduncle of the limb. This 



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