NOTES ON MEDIAN AND PATBED FTNS OF FISH. 335 



of the problem will be dealt with in detail. A brief and some- 

 what dogmatic statement of the case is made at the beginning, 

 followed by a description of my own observations, and ending 

 with a short summary. 



The Lateral Fold Theory. 



Balfour's conception of an originally continuous fm-fold, 

 reaching from the pectoral to the pelvic region (1) is dis- 

 credited because it has been found only (as an epidermal fold) 

 in those forms, like Torpedo, in which the pectoral fins reach 

 the pelvic fins in the adult, a condition Avhich is probably 

 rightly considered as secondary. Moreover, the appearance 

 of an epidermal longitudinal fold, as a first indication of the 

 development of the paired fins, is considered to be of little 

 importance, and its presence between the paired fins is denied 

 in sharks (Mollier24, Brans 4, etc.). 



Now, the continuity of the pectoral with the pelvic fin- 

 fold is not an essential point. The important thing is to 

 recognise that the paired fins alwnys arise as a longi- 

 tudinal fold or ridge, similar to that which gives rise to 

 the median fins. That this is really the case is now admitted 

 by all (Brans 7). Even in Ceratodus, where the paired fins in 

 the adult are set at a pronounced angle to the long axis of 

 the body, they make their first appearance as longitudinal 

 ridges (Semon 33). 



Possibly from the very first, in phylogeny, the paired fins 

 were discontinuous, and differentiated into pectoral and 

 pelvic. For conclusive evidence on this point we must look 

 to palfeontology ; and it has not yet been obtained. But 

 there is some evidence to be gathered from comparative 

 anatomy and embryology in favour of Balfour's view, as has 

 frequently been pointed out (Dohrn 10, Mollier 24). 



For instance, the musculature of the fins is developed in 

 Elasmobranchs, from buds given off from the ventral ends of 

 the myotomes, and these buds have been shown to be pro- 

 duced not only on thcmyotomes in the region of the fins, but 



