6 THE PELAGIC STAGES OE YOUXG FISHES. 



and form either fins or tactile appendages intended for special functions, as 

 in Motella and Lophius among our shore fishes, and among many of the 

 deep-sea fishes, where in addition the comparatively simple lateral organs 

 become greatly developed into special sense appendages. 



From the embryonic fin-folds, by resorption and development of special 

 parts, are formed the caudal, the dorsals, and the anal, the ventrals are 

 developed from the rudimentary embryonic fin fold extending from the vent 

 toward the head along the lower side of the yolk-mass. 



The caudal is usually the first fin to become specialized from the embry- 

 onic fin-fold, though in some eases the ventrals are developed first. The 

 formation of the dorsals and anal goes on pari /hush ; the posterior dorsal, 

 however, is developed usually before the anterior dorsal, when the fins are 

 similar; but when, as in the case of Lophius and Cyclopterus, they form (as 

 well as the ventrals) special organs, as it were, their rudiments are early to 

 appear. The fin-rays of the pectorals make their appearance first ; next 

 come those of the caudals; the dorsal and anal fin-rays are synchronously 

 developed. The delicate embryonic fin-rays of the primitive fin-fold are first 

 formed at the caudal extremity, then gradually extend over the whole of 

 the embryonic fold. The base of the definite fin-rays is the first to appear, 

 and in the early stages these folds are completely covered by a more or 

 less coarse granulation, which also covers the yolk-mass. Mr. Agassiz*has 

 already alluded to the crossopterygian character of the embryonic pectorals 

 of the young of osseous fishes. 



The comparatively large size of the chorda in the earlier embryonic 

 stages is a marked feature of all young fishes. On leaving the egg, the 

 sheath is surrounded by a thin muscular belt, which increases in thickness 

 with the age of the fish. The diagonal muscular bands make their appear- 

 ance early in the embryo, while still in the egg. They are at first most 

 distinct^ marked toward the median part, diminishing in distinctness as 

 they extend toward the head, ami gradually reaching toward the extremity 

 of the tail. With increasing age the large cells of the chorda become sub- 

 divided into smaller ones, and finally assume a granular appearance at the 

 time when the subdivisions of the vertebral column and the first trace of 

 the apophyses appear. 



We have been greatly struck, while making these investigations, by the 

 regularity with which the same stages of development of identical species 



I. Lgassiz. Proc. Am \,-,,a . XVII., L882, |>. 297, PI. XIX. figs. I 6. 



