8 5 6 



THE AMERICAN NATURALIST. [Vol. XXXV II. 



The trunk appendages were represented by the series of 

 fringing processes. 



We may assume that the evolution of the true vertebrates 

 was accompanied by the fusion of the paired mouth parts into 

 unpaired upper and lower jaws, by the further specialization 

 of the gill pouches, the reduction of the free cephalic append- 

 ages to such embryonic structures as certain oral papillae or 

 tentacles, external gills, and the larval balancing organs seen 

 in certain amphibian larvae, and by the fusion of the abdominal 

 appendages to form the lateral fold from which in turn arose 

 the pectoral and pelvic appendages. 



The above interpretation of the origin of paired appendages 

 retains the strong points of both the gill arch, and the lateral 

 fold theories, without the weak points of either. It gives us 

 precisely what Gegenbaur claims has heretofore been lacking in 

 the lateral fold theory, namely: (i) a reason for the existence 

 of the primar\ fold ol ectoderm that initiates the formation of 

 the lateral fold ; (2) a reason for the migration into it of seg- 

 mental detachments of muscle, nerve and cartilage; and (3) a 

 primary function for the lateral fold out of which a set of loco- 



We may explain the presence in the ostracoderms of two or 



one another or with the pectoral fins, without being forced to 

 assume that such highly specialized structures are nothing but 

 movable spines or cornua, or flexible Haps, without any known 

 antecedent function or significance. We may agree with 

 Gegenbaur that there is a certain homology between gill arches 

 and specialized portions of the lateral fold, without assuming the 

 extensive migrations of gill arches demanded by his theory, and 

 we may agree with Dohrn that structures homologous with gills. 



present there. 



considering the shape of the body and its armor, the nature of 

 the appendages, and the position of the eyes and mouth. 



