54 MINOR ABNORMALITIES— RELATING TO FINS 



LOCAL DEFICIENCY OR REDUPLICATION OF THE NOTOCHORD. 



In examining serial sections of trout embryos I have come across three cases of local reduplica- 

 tion of the notochord. In two of them the notochord is bifid at its anterior extremity, becoming 

 single while still in the intra-cranial region. The parachordal cartilages are broad in front and 

 enclose both ends of the notochord. There is no duplicity of any other structure. It is perhaps 

 remarkable that one of these embryos was a cyclops of type B (pp. 41-42). 



The third example of reduplication of the notochord was observed in sections which had been 

 cut from an apparently normal embryo for the purpose of serving as a typical series. In the 

 middle abdominal region the notochord divides into two limbs which lie adjacent to, but quite 

 separate from, one another for four or five segments, and then unite again. Where they are widest 

 apart each has a separate sheath and separate sets of neural and haemal arch cartilages. The 

 adjacent cartilages are disposed exactly as in double monstrosities at the region of transition from 

 the double to the single condition. These cases seem to be examples of local fission affecting a single 

 axial organ, rather than examples of true axial duplicity. 



Local deficiency of the notochord occurred in one specimen. Here the notochord, which is 

 normal in the cranial and cervical regions, ceases abruptly just behind the level of the pectoral fins. 

 After being absent for six somites, it reappears and runs backwards normally along the rest of the 

 trunk. Plate XXVI. fig. 113 illustrates a transverse section in the defective region. The neural 

 and haemal arch cartilages have fused together to form a series of half-rings below the cord. 

 Ventral to these the lateral muscle masses meet one another in a mesial raphe above the dorsal 

 aorta, forming a strong support and sling for the vertebral column and the cord. 



ABNORMALITIES OF FINS. 

 (a) Excess. Examples: 



Salmo irideus, a supernumerary pair of pectorals (Mazzarelli 154); 



Salmo fario, a supernumerary dorsal (Seligmanu 226); 



Salmo fario, two dorsals side by side (observed by the author in a recently hatched embryo) ; 



Siluris glanis, a small supernumerary pelvic almost median in position, but belonging to the 

 right side (Warpachowski 267); 



Pleuroncctes platessa, a well-formed four-rayed fin near the middle line just behind the two 

 normal six-rayed pelvic fins (Duncker 59 376-8) ; 



Pleuroncctes platessa, an abnormal precaudal fin frill on the left (i.e. under) side (Boulenger 30) 

 and M'Intosh (155); 



Acanthias vidgaris, an almost median accessory fin on the top of the head (Grosser and 

 Przibram 85) ; 



Raia clavata, an upright, oval-shaped dorsal fin in middle of back (Day 51^ II. p. 344) ; a pair 

 of rudimentary pectoral fins on the back (Gervais 80); 



Raia batis, two examples of accessory fins, both being of the paired-fin type (Rennie 203). 



In various goldfish races there occur: (1) excessive growth of a fin or portion of a fin, and (2) 

 partial or complete doubling of certain fins, e.g. of the caudals and anals. The latter condition may 

 be accompanied by doubling of the fin-ray supports and of the inferior spinous processes of the last 

 two vertebrae (see Bateson 19 pp. 451-454). 0. Storch (24-Oa), who describes in detad the 

 muscular and skeletal elements of the paired anal fins in the "Binkin" variety of goldfish, 

 argues that we have here the key to the evolution of the true paired fins. In this view these 

 fins have not taken origin from lateral fin membranes, but have arisen by the splitting of 

 portions of a median fin, while the limb-girdles have been evolved in response to the new 

 dynamical conditions at the bases of these fins. 



The lung-fishes supply a number of examples, among which may be noted : 



Protopterus, a bifid right pectoral (Albrecht 1) ; a trifid left pectoral (Boulenger 28, Hopley 98) ; 



