ON THE CAUDAL AND ANAL FINS OF GOLD-FISHES. 



259 



ectodermal cells forming a dorsal median thickening constitutes the 

 beginning of the dorsal fin-fold. This fold extends backward and 

 around the caudal termination and passes over to the double primor- 

 dial ridge (/,'/') on the ventral side. 



With the growth of the embryo, the ridges or folds grow in 

 height. The dorsal fold extends anteriorly as far as the level of 

 the eyes. The two ventral ridges behind the anus diverge from each 

 other at a later period as shown in Figs. 28 and 29. 



Still later, at two regions, one behind the anus and the other at 

 the hind end of the body, the folds greatly develope and become 

 markedly broad, specially at the latter rt.gion as represented in Fig. 33. 



The portion lying between these two local thickenings remains 

 undeveloped and becomes finally atrophied. 



The thickenings ( a.'f.) just behind the anus are the rudiments 

 of anal fins and the hind ones (c.'f.), those of caudal fins. 



Fig. 29 represents an embryo of this stage as seen from the 

 ventral side. This state of the ventral fold gives rise to the paired 

 anal and caudal fins. In other cases, the paired portion is restricted to 

 the region just behind the anus, the remaining portion being unpaired 

 (Fig. 31). Such a state would give rise to a form in which the anal 

 fin alone is paired. In others again, it is in the caudal extremity 

 alone that the fold is double (Fig. 30). From such an instance there 

 arises the form with the caudal fin alone paired, while the anal 

 remains normal. 



The varying extent to which the ventral portion of the pri- 

 mordial fin-fold is cleft is the cause of all those intermediate fins 

 between the paired and the unpaired state. Fig. 32 represents a case 

 in which only a shallow groove runs along the ventral margin of the 

 fin-fold. Such an embryo, for instance, would give rise to fins which 

 are only partially double. 



