420 The American Naturalist. [May, 
Ryder has given avery full account of the embryology of 
fishes’ fins, which has especial reference to the caudal region.” 
He distinguishes six stages in the development of the caudal fin 
as follows: 
1. Archicercy; no caudal fin-folds. 2. Lophocercy; caudal 
fin folds, with or without actinotrichia. 3. Diphycercy; 4, Het- 
erocercy ; and 5, Homocercy, as above defined. 6. Gephyro- 
cercy, in which the terminal vertebrz are aborted, so that a 
hiatus is created between the neural and hzmal elements, or actino- 
phores, at the extremity of the axis. This structure occurs in 
Echiodon (Pl. xvii., Fig. 3,) Mola, etc. In this case the epaxial ; 
and hypaxial tegumentary folds are continuous round the ex- 
tremity of the vertebral axis, and develop fin-rays which appear 
later than those of the dorsal and anal fins. This is supposed to 
be a condition of degeneracy.. 
The subject is treated histologically and embryologically by 
Ryder, who reviews the work of various authors who have viewed 
it from the same standpoint, especially Vogt, Kölliker, Dohrn, 
and Lotz. 
The latest contribution to the subject is one by Baur, who 
shows that the rods (axonosts) which connect the rays of the anal 
fin with the inferior side of the caudal vertebrz, in Lepidosteus, are 
chevron bones. This agrees with the determination by Cope” 
that the vertebral bodies of fishes are intercentra, and not centra. 
From the researches already cited it is now well understood 
that in the caudal fin as in other fins, the primitive actinotrichia 
have become specialized into fin-rays, and that these approximate 
more nearly to the number of their osseous supports than do the 
actinotrichia. They are rarely so reduced in number, however, 
as to correspond exactly to the hypural bones. 
The actinophores of the caudal fin may or may not have been 
primitively divided like those of the other median fins into 
35 Annual Report of the U.S. Commissioner of Fishes and Fisheries, 1886, pp. 981- 
1086, 
_ 86 Amer, Journal of Morphology, 1889, p. 463. Dr. Baur inadvertently calls the axo- 
nosts “ actinosts.”’ 
31 Transac. Amer. Philosoph. Society, 1886, p. 243. 


