TRANSACTIONS OF SECTION D. t)97 



11. Points in the Development of the Pectoral Fin and Girdle in Teleosteans. 



By Edward E. Pkince. 



At a Tery early stage, and long before liberation from the ovum, the pectoral 

 fins can be distinguished in Osseous Fishes as a pair of flattened pads projecting 

 horizontally from the trunk, some distance behind the otocysts. Their position 

 seems to vary in different species,^ but a considerable interval always separates the 

 early fins from the auditory organs, or rather from the true pectoral region. They 

 are differentiations of a continuous lateral expansion of epiblast passing along each 

 side of the trunk, and are formed by the folding of this epiblastic layer upon itself 

 at the point where the fins appear. Each fin consists therefore of two epiblastic 

 lamellcB (separated by a fissure) lying flat upon the vitellus, and continuous with 

 the extra-embryonic blastodermic membrane. The fins soon assume a denser 

 appearance as mesoblastic cells push their way into the median fissure, separating 

 the upper from the lower lamella of the fin. These mesoblastic cells are certainly 

 not splanchnopleuric, but as no well-marked somatopleuric crest has been recognised 

 in fishes comparable to the Wolffian ridge of higher forms, they seem to be derived 

 from the intermediate cell-mass in close proximity to the Wolffian ducts. 



As the fin becomes stouter its position alters, its lengthy lateral attachment to 

 the trunk diminishes, while it becomes pedunculate and stands erect, though not 

 quite vertically. The distal portion of the fin is very thin and transparent, save at 

 the periphery, where, at the junction of the upper and lower lamella, the epiblast 

 cells are approximated so as to form a marginal ridge probably connected with 

 the subsequent development of dermal fin-rays. 



Meanwhile the fin progresses over the interval before mentioned to a point 

 immediately behind the auditory organs, and the well-known rotation of the fin is 

 accomplished, so that it now projects obliquely in a dorso-ventral direction almost 

 parallel to the plane of the branchial arches in front. 



The gradual shifting of the fin from its original place brings it into close relation 

 with a band of mesoblastic cells, which passes obliquely behind the otocysts, and is 

 called by Ryder the ' oblique or vertical pectoral fold.' In this fold cartilage-cells 

 appear, and extend dorsally and ventrally. To the cartilaginous pectoral bar, thus 

 formed, the fin-plate becomes attached, and simultaneously a median stratum of 

 mesoblast in the latter is converted into cartilage, which, by its basal portion, 

 articulates with the girdle. It is noteworthy that each half of the pectoral arch 

 .originates independently, nor do' they approach each other in the middle ventral 

 line until a much later stage. A strong plate of translucent ectochondral bone, 

 like a curved bar of chitin, develops and becomes attached to the scapulo-coracoid 

 rod, and the subsequent reduction of the latter (the primary cartilaginous girdle) 

 produces much complication in the adult structure. 



Without desiring to lay undue stress upon the suggestions afforded by the 

 development of the anterior limb and its girdle in forms so highly specialised as 

 the Osseous Fishes, it is still of interest to note their bearing upon accepted theories 

 as to the true nature of the fin and its related arch. 



Support is thus certainly given to the continuous lateral-fin theory of Balfour, 

 for the fins arise in connection with longitudinal epiblastic ridges extending in a 

 horizontal plane ^ fi-om the trunk of the embryo, while their independent origin is 

 adverse to Gegenbaur's view that they spring from a branchial arch and are 

 modified gill-arch elements. Owen, more than a quarter of a century ago, com- 

 pared the branchiostegal rays springing from the hyoid arch to the ' pectoral fin 

 aiverging from the htemal arch ' {i.e., the pectoral girdle) ; but Balfour's view (which 

 is also that of Dohrn and Mivart) receives more countenance from Teleostean 

 embryology. Dohrn, however, not only regards the fins as aggregations of a long 

 lateral fin ; but supposes that the coalesced fins became connected with underlying 

 gill-arches which ancestrally extended beyond their present limits. Gegenbaur 



' The forms especially referred to by the writer are certain species of Gadvs, 

 Pleuronectes, Cottus, &c., studied at the Marine Laboratory, St. Andrews. 



- This position is, according to Gegenbaur's view, secondary, whereas in Teleo- 

 steans it seems to be primary, and the vertical position is assumed secondarily. 



