J. K. Thacher — Median and Paired Fins. 299 



cartilages, and the derivation of tlie latter from the lateral folds of 

 Amphioxns. 



Though the last mentioned portion of the theory would derive 

 considerable strength from the establishment of the first, it is not a 

 necessary consequence of it, and the first might be true even if the 

 last were false. If the last be true, of course the first must be true. 



The establishment of the derivation from the lateral folds of 

 Ainphioxus is made difficult from the al)senee of limbs or anything 

 representing them in the two groups which (in a sense) stand between 

 Amphioxus and the Elasmobranchs, namely, the Myxinoids and 

 Lampreys. 



As will be seen, it assumes the essential correctness of Huxley's 

 suggestion with regard to the relation between the folds which grow 

 down to inclose the atrial cavity of Amphioxus and the body walls 

 of higher vertebrates. But it is equally consistent with Huxley's* 

 entire suggestion, as put forth by him, or with Ray Lankester'sf 

 modification of it. 



On the other hand, it is inconsistent with Kowalewsky'sJ view of 

 the homology between these and opercular folds. This must perhaps 

 be consideied still an open question, though Rolph's§ arguments on 

 the other side seem to me of much less weight than they do to 

 Semper. II 



Yet even if this homology with the lateral folds should have to be 

 giVen up (the embryology of the Marsipobranchs will throw consid- 

 erable light on it), the very frequent occurrence of the formation of 

 external lateral folds parallel to the axis of the body in the bilateral 

 animals in general and in the Vertebrates in particular, renders it 

 quite possible that the paired fins may have had a similar origin. 

 At present, however, I am strongly of the opinion that they are 



* Joum. of Linn. Soc, vol. xii, No. 59, May, 1875. Huxley, Classification of the 

 Animal Kingdom. 



f Quarterly Journ. of Micr. Sc, New series, No. 59, July, 1875. Ray Lankester, 

 New Points in the Structure of Ampliioxus. 



X Mem. St. Petersb. Acad., VII Series, tome xi. No. 4, 1867. A. Kowalewsky, 

 Entwickelungsgeschichte des A mphioxus lanceolatus. 



§ Sitzungsberichte der Naturforschenden Gesellschaft zu Leipzig. .Tahrg. II, No. 1, 

 Jan. 29, 1875. Rolph. Unters. viber den Ban des Amphioxus lanceolatus. See also 

 for a complete account of his investigations, under .,he same title, Morph. Jahb., Bd., 

 ii, Hft. 1, 1876. 



II C. Semper, Die Verwandtschaftsbeziehungen der gegiiederten Thiere, 1875, p. 

 317. (Sep. Abdr. aus Semper: Arbeiten a. d. Zoolog-zootom Institut zu Wurzburg, 

 Bd. II). 



