v FIN-BAYS '349 



surface so that its diameter becomes increased. In the fins of 

 Elasmobranchs and in the adipose fins of Teleosts these horny fin- 

 rays become much elongated, but ordinarily in the Teleostome they 

 become relatively shortened during development, apparently being 

 absorbed at their proximal ends while they grow at their distal 

 ends, so that in the fully developed fin they form merely a marginal 

 fringe — the individual horny rays being concentrated about the ends 

 of the lepidotrichia. 



The fin-rays in question have the main feature in common with 

 the lepidotrichia that they arise in very close relation to the base- 

 ment membrane and later on separate from it and sink into the 

 underlying mesenchyme. It is probably allowable to look upon 

 them as being of the same nature morphologically as the lepidotrichia 

 but as having evolved still farther from the primitive scale-like 

 condition. In Lung-fishes horny fin-rays occur of somewhat inter- 

 mediate character in the form of slender parallel rods irregularly 

 jointed and branched distally. In the later stages of their develop- 

 ment these rays are apt to assume a bony character, becoming strongly 

 calcified and enclosing branched bone corpuscles in their substance. 

 The early development of these rays has not, so far, been worked out 

 in Ceratodus in which they are best developed. In Protopterus 

 Goodrich found them in early stages in at least very close proximity 

 to the basement membrane. 



We may probably look upon the dermal fin-rays of fishes as 

 belonging to one morphological category, representing structures of 

 originally placoid nature which have sunk down into the mesen- 

 chyme and degenerated, or — to use a preferable expression — become 

 specialized, into' more or less horny structures. The earliest stage 

 in this process we should see in the lepidotrichia of Teleostomatous 

 fishes where the scaly nature is still quite clear. The fin-rays of Lung- 

 fishes would represent a farther stage, in which the scale homology is 

 no longer clear, and the horny rays of Elasmobranchs and Teleostomes 

 would represent the final stage, in which all trace of the original 

 nature had disappeared except the origin in close association with 

 the basement membrane. 



The lepidotrichia of the Teleost and its horny fin-rays would 

 represent different generations, the lepidotrichia being a later genera- 

 tion which have, as it were, had less time for modification. In this 

 connexion it should be mentioned that the horny fin-rays of the 

 Elasmobranch do not all belong to one generation. Their production 

 from the basement membrane may go on for a long period so that 

 instead of a single layer a thick mass of rays may be developed. 



SKELETON OF THE PAIRED LIMBS 



I. Fins. — To be consistent with the plan adopted in this book we 

 should commence with the development of the paired fin in its most 

 nearly primitive existing form. Before this can be done it is necessary 



