988 EEPORT OF COMMISSIONER OF FISH AND FISHERIES. [8] 



the adult. Beyond the lophocercal condition the typical fishes at once 

 diverge from the rest of the (Jhordata and Vrochorda, in that they de- 

 velop groups of permanent rays in definite regions of the median fin- 

 fold or continuously throughout its entire extent, and thus give rise to 

 the distinct or continuous fins of the adult. The intervening parts of 

 the fold in the first case atrophy, thus locally reverting to archicercy, 

 the materials for the formation of the rays and their supports being- 

 supplied by the mesoblast which ])roliferates into the median fin-fold. 

 The disposition of the materials for the development of the permanent 

 rays of the unpaired fins seems to be under the control of heredity, 

 which determines their permanent location or position in the primitive 

 folds, which may be considered the matrix of the permanent fins. 



In the formation of rays and their supports and musculature there 

 is clearly a close correspondence between the number of ray-bearing 

 somites of the body and the one, two, or three rays and supi)orts which 

 are develo])ed to each segn)ent, and this is manifested even when heter- 

 ocenty and its accompanying degenerative processes manifest themselves 

 in the cau(l;il region of tlie most specialized forms. AYe find, in fact, 

 that where apparent coalescence of two hypural elements has occurred, 

 two corresponding mesoblastic thickenings of the caudal fold are de- 

 veloped, which extend from the lower end of the hypural pieces to the 

 margin of the fold, and in and upon which the future i)ermaneut ray is 

 molded. Tliis segmental proliferation of mesoblast is possibly favored 

 by the intersegmental disi)osition of the blood-vessels. 



(3.) Diphi/ccrcTj. — The most, prnnitive disposition of the median fin- 

 rays is a continuous one, as is indicated by the embryological evidence 

 of the existence of a multiradiate protopterygian stage, and is hypaxial 

 from in front of the vent in many cases, extending backward ovei- the 

 end of the tail, thence epaxially and forward dorssdly [Gcelacanthi, Placo- 

 dermi.f IJipnoi, Fig. 4). Another archaic trait which also marks a phase 

 of the ontogeny of the Teleosts is the co'hujan- 

 thus (hollow) condition of the bony portion of 

 the neural and luemal spines and the interspinous 

 elements. Fishes with a long, eel-like body 

 have tended to remain diphycercal, while those 

 whose bodies have been abbreviated have 

 tended, with the exception of such forms as the 

 Flounders, or Hefcrosomata,, to develop discontinuous median fi.ns, Avhich 

 have been derived from hypertrophied portions of a continuous series. 

 This hypertroph;*;' in some cases involved the whole series, e.g., Flata.r. 

 The priuKTeval diphycercal condition is followed by the next stage, 

 which grows out of the former in the course of further local degenera- 

 tion and specialization, Mith a secondary upbendiug of the hinder end 

 of the chorda. This view is fully substantiated by the development of 

 the caudal skeleton of the eel, in which, in spite of its slight heter- 

 ocercy, the diphycercal continuity of the tin series has remained prac- 



