G4 



FORMATION OF THE TAIL. 



In the Flounder, winch may serve as a type, the primitive sym- 

 metry is very soon destroyed by the appearance of fin rays on the 

 ventral side. The region where they are present soon forms a lobe ; 

 and an externally heterocercal tail is produced (fig. 36 A). The 

 ventral lobe with its rays continues to grow more prominent and 

 causes the tail fin to become bilobed (fig. 36 B) ; there being a dorsal 

 embryonic lobe without fin rays (c), which contains the notochord, and 

 a ventral lobe with fin rays, which will form the permanent caudal fin. 

 In this condition the tail fin resembles the usual Elasmobranch 



form or still more that of some 

 Ganoids, e.g. the Sturgeon. The 

 ventral lobe continues to develop ; 

 and soon projects beyond the dorsal, 

 which gradually atrophies together 

 with the notochord contained in it, 

 and finally disappears leaving hardly 

 a trace on the dorsal side of the 

 tail (fig. 36 C, c). In the mean- 

 time the fin rays of the ventral 

 lobe gradually become parallel to 

 the axis of the body ; and this lobe, 

 together with a few accessory dorsal 

 and ventral fin rays supported by 

 neural and haemal processes, forms 

 the permanent tail fin, which though 

 internally unsy in metrical, assumes 

 an externally symmetrical form. 

 The upturned end of the notochord 

 which was originally continued into 

 the primitive dorsal lobe becomes 

 ensheathed in a bone without a 

 division into separate vertebra?. 

 Thisbone forms the urostyle(). The 

 hffimal processes belonging to it are 

 represented by two cartilaginous 

 masses, which subsequently ossify, 

 forming the hypural bones, and 

 supporting the primary fin rays of 

 the tail (fig. 36 C). the ultimate 

 changes of the notochord and uro- 

 style vary very considerably in the 

 different types of Teleostei. Tele- 

 ostei may fairly be described as 

 passing through an Elasmobranch 



FIG. 36. THBEE STAGES IN THE DE- 

 VELOPMENT OF THE TAIL OF THE FLOUNDER 



(PLEUBONECTES). (After Agassiz.) 



A. Stage in which the permanent 

 caudal fin has commenced to be \isible 

 as an enlargement of the ventral side of 

 the embryonic caudal fin. 



B. Ganoid-like stage in which there 

 is a true external heterocercal tail. 



C. Stage in which the embryonic 

 caudal fin has almost completely atro- 

 phied. 



c. embryonic caudal fin ; /. perma- 

 nent caudal fin; n. iiotochord ; u. uro- 

 style. 



stage or a stage like that of most 



pre-jurassic Ganoids or the Stur- 

 geon as far as concerns their caudal fin. 



The anterior paired fins arise before the posterior; and there do 



