TELEOSTET. 



79 



B. 



In some few cases, e.g. Gadus, Salmo, owing to the simultane- 

 ous appearance of a number of fin rays on the dorsal and ventral 

 side of the notochord the external symmetry of the tail is not 

 interfered with in the above processes. In most instances this is 

 far from being the case. 



In the Flounder, which may serve as a type, the primitive 

 symmetry is very soon destroyed by the appearance of fin rays 

 on the ventral side. The re- 

 gion where they are present 

 soon forms a lobe ; and an 

 externally heterocercal tail is 

 produced (fig. 36 A). The 

 ventral lobe with its rays con- 

 tinues to grow more promi- 

 nent and causes the tail fin to 

 become bilobed (fig. 36 B) ; 

 there being a dorsal embry- 

 onic lobe without fin rays (c), 

 which contains the notochord, 

 and a ventral lobe with fin 

 rays, which will form the per- 

 manent 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 gra- 

 dually 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 meantime the fin rays 

 of the ventral lobe gradually 

 become parallel to the axis of 

 the body ; and this lobe, to- 

 gether with a few accessory dorsal and ventral fin rays supported 



THREE STAGES IN THE DE- 

 OF THE TAIL OF THE 

 (PLEURONECTES). (After 



FIG. 36. 



VELOPMENT 

 FLOUNDER 

 Agassiz. ) 



A. Stage in which the permanent 

 caudal fin has commenced to be visible 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 ; f. permanent 

 caudal fin ; n. notochord ; it. urostyle. 



