1885.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 157 



of the yelk-sac, the ventral part of the fold being interrupted only by 

 the posterior part of the gut at v. Fig. 1. This fold at the time of 

 hatching contains simple embryonic rays throughout its entire extent, 

 these being indicated in Fig. 1 by the fine linear striation apparent for 

 the whole length of the fold in the engraving. These embryonic rays, 

 as they have been called by A. Agassiz, I will call actinotricJna, from 

 their slender, unsegmented, hair-like appearance: they are developed 

 from special cells of the mesoblast, which I have named pterygohlasts 

 elsewhere. These actinotrichia exceed in number the permanent rays 

 of the adult at least ten-fold. The latter are in fact in great part 

 formed by the fusion of a number of these actinotrichia lying side by 

 side. 



The extent to which actinotrichia are developed in the median fin -fold 

 at the time of hatching, however, varies very greatly in different genera. 

 In the recently hatched salmon, which passes through a prolonged pe- 

 riod of incubation, these rudiments of the future permanent rays are 

 very numerous, far more so than is the case with most other forms at 

 the time the embryo escapes from the egg. In the Spanish mackerel 

 and in Qadus the actinotrichia are not well defined until sometime after 

 hatching, and then only in the posterior end of the median tail-fold and in 

 the pectoral fin-fold. As a rule, actinotrichia, (horny -fibers of Balfour), 

 appear first in the anterior paired fins ; this is the case in the salmon, 

 in which these primitive rays are well marked in the pectoral about the 

 time the ventral fin-folds vt become well defined. 



As urged in my paper " On the origin of heterocercy," the presence 

 of actinotrichia throughout the whole extent of the fin-folds of the em- 

 bryos of the salmon is an illustration of the Hseckelian principle, viz, 

 that the ontogeny of a form is usually an epitome of the phylogeny of 

 the same. The persistence of the pneumatic duct and the presence of 

 adipose fins are also to be considered in this connection, since both are 

 archaic characters, the first especially. 



The permanent dorsal rays of the salmon are formed from the actino- 

 trichia developed in the anterior part of the median fold d.,fig. ], to- 

 wards which radial muscles are shoved out at an early stage, as shown 

 by the evenly stippled intervals at the base of this part of the fold. 

 The fold in the interval between the dorsal and soft dorsal atrophies 

 with the further growth of the young fish, the actinotrichia of this 

 interval also disappearing with the fold. 



The next portion sd of the median vertical fold which is perceptibly 

 widened, gives rise to the soft dorsal or " adipose fin." The aotiuo- 

 trichia of this fin never pass beyond their embryonic condition, so that 

 it is said by the comparative anatomists to contain horny fibers. The 

 whole of the Plectospondyli except the Characinidcc have lost their adi- 

 pose fins, and thus have but one dorsal remaining. The herrings and 

 salraonoids or Isospondyli would therefore seem to stand in an ancestral 

 relation to the carps, suckers, and minnows, or Plectospondyli. It is at 



