520 EEPORT OF COMMISSIONEE OF FISH AND FISHERIES. [66] 



longitudinally expanding the portion outside of the inner contour line 

 of the fin, as shown in Fig. 40, into a fan-shape, so that the whole fin 

 becomes much more distinctly pedunculated as viewed from the side. 

 This distal thinner portion is at first without any evidence of lays, 

 further than that there is a manifest tendency to a radial disposition of 

 the histological elements of the fin. This radial disposition of the 

 histological elements of the fin-substance has an undoubted relation to 

 the growth in length and expansion of the organ and is conspicuously 

 manifested in the development of the dermal lobes of the caudal fin of 

 Alosa before the development of rays. The distal lamina, as we may 

 call the thinner extremity of the fin, is the portion in which the rays are 

 formed, while the thicker proximal or basal portion is that in which the 

 basal elements of the fin are developed. Just at the point where the 

 basal portion of the fin joins on to the body there is a decided fold ex- 

 tending up and down obliquely on the sides of the embryo and con- 

 tinuous with the fin ; this may be called the oblique or vertical pectoral 

 fold ; just at the base of the fin and in this fold the coraco-scapular 

 cartilage makes its appearance as a somewhat L-shaped i)late, with its 

 anterior coracoid limb extending forwards and downwards and its upper 

 and scapular limb extending upwards. This is the first rudiment of the 

 shoulder-girdle ; the membrane bones which develop around it after- 

 wards ectosteally, appear much later than in the stages so far described. 

 The coraco-scapular cartilage I have studied most successfully in entire 

 embryos of Alosa hardened in picric acid, cleared in oil of cloves, and 

 mounted entire in Canada balsam. 



As regards the detailed history of the development of the various 

 ossicles of the pectoral. fin 1 have little or nothing to record, further 

 than to say that there is no evidence of a type of development like that 

 seen in Elasmobranchs; the evolution of the breast fin of Teleostei be- 

 ing influenced by the specialized character of the limb-skeleton of the 

 adult. The muscles are developed as in the Elasmobranch fin from 

 mesoblastic strata of cells internal and external to the median plate, 

 from which the cartilaginous axial portions of the fin are evolved. 

 Sections through the pectorals of the larvse of Oambusia show the de- 

 tails of muscular development to be very similar to that represented 

 by Balfour in Fig. 346 (Comp. Embryol., 11), as obtaining in ScylUum. 



The next points of interest in this connection are the changes of po- 

 sition which the pectorals undergo in relation to the surrounding struct- 

 ures and their rotation upon their bases, by which they acquire an up- 

 right position so as to become mechanically effective as organs of pro- 

 pulsion or locomotion. At first quite longitudinal in direction, as shown 

 in Figs. 32, 34, 40, and 42, the anterior portion tends to be gradually 

 elevated as development proceeds, its base becoming more or less ob- 

 lique in position as viewed from the side. Finally this process of the 

 rotation of the base is carried so far that the fin acquires a nearly or al- 

 together vertical position on the side of the body. The face of the fold 



