368 AETHUR WILLEY. 



pleura had been entirely different in their nature from the 

 median fins it is not very likely that one of them would have 

 undergone concrescence with the ventral fin. As shown by 

 Lankester and Willey, the metapleural folds arise as solid 

 longitudinal thickenings of the integument, which are at first 

 largely ectodermic in origin (the ectoderm-cells assuming a 

 columnar character), while the cutis subsequently takes part in 

 their formation. Eventually a lumen (schizocoele) appears in 

 the ridges. 



The right metapleuron is in advance of the left in order of 

 appearance, and in front of the pharyngeal region of the larva 

 it curves sharply inwards towards the middle line, in which it 

 gradually dies avray on the ventral surface of the snout. 



There is, in fact, no essential difference between the mode 

 of origin of the metapleura and of the median fins as integu- 

 mentary ridges, and it is possible that in the above-mentioned 

 cases, in which the right metapleuron is continuous with the 

 ventral fin [i. e. the mesial ridge in connection with it), they 

 actually arose in continuity in the first instance. 



If, then, it is necessary to admit the intrinsic similarity in 

 the nature of the metapleural folds and the median fins of Am- 

 phioxus, we are led back to the theory of Thacher with 

 reference to the origin of the paired limbs of Vertebrates. 



Balfour, as is well known, was the first to discover the con- 

 tinuous lateral fin-ridges of the Selachian embryo in 1876; 

 and it is curious to note, in the light of what has just been 

 said as to the ectodermic thickenings which prelude the 

 formation of the metapleura in Amphioxus, that they also [i. e. 

 the Selachian fin-ridges) consist in longitudinal thickenings of 

 the epiblast. From his observations on the embryonic de- 

 velopment of the Selachians, Balfour came to the conclusion 

 " that the limbs are the remnants of continuous lateral fins ;" 

 but he did not suppose that the continuous lateral fins were 

 represented in Amphioxus. 



At about the same time, and quite independently, Thacher 

 was led by observations on the adult forms and on the skeleton 

 of Selachians and Ganoids, &c., to a belief in the homodynamy 



