The Embryology of Heterodontus japonicus 



759 



plan of these gill-filaments is not unlike that described for Chlamydoselachus (Smith 1937). 

 Each filament of Heterodontus is a narrow band attached by one edge to the gill-septum 

 which it traverses in a radial direction. Thus the filaments lie approximately parallel to 

 one another; they are very numerous and are set close together. The extreme distal end 

 of each filament stands slightly away from the gill-septum; in other words, the distal ends 

 are free from direct attachment to the septum. Each filament bears on both surfaces 

 a series of close-set parallel ridges, the lamellae, which extend transversely to the long 

 axis of the filament. In Heterodontus the close-set lamellae project slightly beyond the 

 free edge of the filament, giving it a serrated appearance. 



In the 280-mm. (11-inch) young female Heterodontus japonicus the gill-filaments are 

 much the same as in the adult specimen of H. quoyi; but the filaments are longer and their 

 distal extremities project farther from the gill-septum. Except for their serrated appear- 

 ance, these finger-shaped extremities of the gill-filaments suggest the rudimentary filaments 

 of the early embryo. The filaments of this specimen are easily exposed, since they cover 

 a considerable extent of the gill-septum. They are not, however, visible when the gill- 

 flap is closed. 



In the 78-mm. (three-inch) embryo of H. japonicus already mentioned, I found the 

 rudiments of the internal gill-filaments (those that persist in the adult) in connection with 

 external filaments (shown in Figure 38, plate III). The two kinds of gill-filaments are 

 continuous structures. The rudimentary internal filaments are attached, throughout all 

 but a small distal portion, to the gill-septum and are distinguished by the presence of 

 rudimentary lamellae. The distal ends of these internal filaments are continuous with the 

 rod-like external gill-filaments, which lack lamellae. 



Bearing in mind this relationship between the external gill-filaments of the embryo 

 and the internal gill-filaments of the adult, the occurrence of gill-filaments protruding from 

 the spiracular cleft is conclusive evidence (if such evidence were needed) that the spiracu' 

 lar cleft in sharks was primitively a gill-cleft functioning in the usual manner. During 

 early development the spiracular cleft is as large as the 

 gill-clefts, with which it is serially homologous; but dur- 

 ing later development its external orifice becomes very 

 small, and the spiracular canal takes on special functions 

 concerned with respiration. In some specimens of Heter- 

 odontus the spiracle is so small that it seems vestigial. 



Text-figure 66 



Roof of the mouth cavity of a 78-mm. (3-inch) embryo of Heter- 

 odontus japonicus. The dental ridge, formed by the upper jaw, 

 is situated between the olfactory region anteriorly and the breath- 

 ing valve posteriorly. The small pit, shown in the center of the 

 figure, leads into Rathke's pouch. 

 From a drawing left by Bashford Dean. 



