350 PROCEEDINGS OF THE ACADEMY OF , [1887. 



The less marked development or thickening of the somatopleure 

 over the lateral and ventral parietes of the body-cavity, in the prim- 

 itive vertebrates, would doubtless tend to affect the position of the 

 notopodia, from which the paired limbs are supposed to be derived, 

 causing them to retain their j^rimitive place. 



Thus far, only the ontogenetic theory of the limljs has been dis- 

 cussed, in order to prepare the reader for the evidence which is now 

 to be presented ; many of the data are from personal studies, and 

 such old data as I have found available, I trust, will be brought 

 into such contrast with the others, as a whole, as to bring out their 

 morphological significance. 



II. The jjroof of the inexact homology of the limbs of different types. 

 The inexact homology or equivalency of the limbs of different 

 types of vertebrates has been suspected by authors, but no observa- 

 tions or systematic comparisons have been put upon record to show 

 that there were good morphological grounds for such a conception 

 of the nature of limbs. The exact morphological equivalency of 

 the pairs of limbs of different types, would require that they arise 

 from the sides of the same segments or somites behind the skull, for 

 each pair, in all sjDecies of vertebrates. If, for example, the limb- 

 bud of the pectoral or fore-limb arose from the fifth to the eighth 

 post-occipital somites in all vertebrates, there could be no question 

 that the fore-limb in one was the exact homologue of the fore-limb 

 in every other form. Unfortunately, the facts of development and 

 comparative morphology unequivocally compel us to admit that 

 such is not the case. 



1. Emhryological evidence, considering especially the points of origin 

 of paired limbs. 



The positions in which the limb buds of the same pair ai'ise in 

 ditferent types varies between wide limits, and, while there are causes 

 which in part explain these variations, there are some complications 

 involved which refuse any other explanation except that which sup- 

 poses, that such variation in point of origin is an indication of in- 

 exact homology. 



It may be stated as a general truth that, the paired nerves which 

 go to a given limb have arisen from the somites, which were oppo- 

 site or beneath the limb bud or fold, from which that limb developed. 

 This may be shown to be true in the lower forms nearest the primi- 

 tive type, from which all others may be supposed to have arisen. 

 For example, the pelvic pair of limbs of physoclistous Fishes at first 



