INTRODUCTORY 



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spines of the vertebrae in front of this are enlarged, those at the 

 lower angle of the tail being longer so that the posterior edges 

 of these bones are in a vertical line, and the dermal rays artic- 

 ulating with these posterior edges form a fin which is out- 

 wardly symmetrical although all the rays except a few small 

 ones at the upper angle are below the axis of the vertebral 

 a h 



Fig. 21. — Development of the homocercal tail as seen in a young flat-fish- 

 After Alex. Agassiz. a, youngest stage, almost symmetrical, without fin-rays ; 



b, end of notochord slightly up-turned, dermal fin-rays appearing on ventral side ; 



c, d, end of tail more bent up and reduced : e, f, the ventral rays become 

 terminal. 



column : a tail of this kind is described as homocercal, and is 

 seen in Salmon, Gold-fish or Mackerel. In the development 

 of a bony fish from the newly-hatched stage or larva, the tail- 

 fin passes through stages which repeat the stages of its evolu- 

 tion (Fig. 21) ; the first signs of skeletal structures in the mem- 

 brane are delicate hair-like rods representing the horny fibres 

 of the dog-fish, then the end of the notochord becomes bent up 

 and the fin-rays begin to appear on the lower or ventral side, 

 thus forming a heterocercal tail (b.) ; then by the growth of the 



