The Embryology of Heterodontus japonicus 747 



greatly increased. The fins, excepting the caudal, are larger than in the preceding figure, 

 and all the fins show an advance in differentiation. The tips of the spines of the dorsal 

 fins are now exposed, and the ventral lobe of the caudal fin exhibits the notch that is 

 characteristic of adults of the genus Heterodontus. The lateral line, extending along the 

 side of the body, is fairly distinct, and portions of the sensory canal system of the head 

 are indicated by white Hnes in the drawing. The external gill-filaments are shorter, more 

 delicate, and perhaps less numerous, than in the preceding stage. Seven or eight filaments 

 project from the spiracular cleft. The demibranch on the anterior side of the first branchial 

 cleft shows some of the shortened gill-filaments that persist in the adult. In this drawing, 

 the supraorbital fold would scarcely be noticed if one were not familiar with its form 

 in the adult fish. 



One of Dean's embryos is about 72 mm. (2.8 inches) long. It is slightly less advanced 

 in its general development than the one represented in Figure 38, plate III, but has 

 external gills that resemble those shown in this figure. Another embryo in Dean's 

 collection, apparently identical with the one depicted in Figure 38, plate III, is about 

 78 mm. (3 inches) long. 



Figure 39, plate III, represents an embryo definitely older than the preceding. This 

 is shown by the emergence of several new features. The body proper, and the bases of 

 certain fins, are covered with dermal denticles. The color pattern of the embryo at the 

 time of hatching is vaguely foreshadowed. The dorsal fins have acquired somewhat their 

 form in the adult, though in adults of this species the posterior margin of the second dorsal 

 is sometimes partly or wholly convex. The tips of the spines of the dorsal fins are barely 

 visible. If one looks sharply he may see, just behind the gill-clefts and continuing caudad 

 for some distance, a series of grooves parallel to the gill-clefts and spaced Uke them, but 

 not so distinct. These grooves are better shown in Figure 84, plate VII, considered 

 later. The supraorbital ridges are hardly noticeable and are perhaps not well develop- 

 ed in this embryo. The general form of the body, now approximately the same as in the 

 adult, differs Httle from that represented in the preceding figure. The spiracular 

 opening is not shown. 



An embryo in Dean's collection, apparently identical with the one represented in 

 Figure 39, plate III, is broken in two in the middle and is somewhat mutilated in this 

 region. It cannot be accurately measured but is about 90 mm. (3.5 inches) long. 



The embryo depicted (in color) in Figure 82, plate VII, is a little older than the one 

 represented in Figure 39, plate III. The presence of a sizable yolk sac shows that it was 

 taken long before hatching. In some respects, this embryo is unique and is probably 

 either distorted, abnormal or inaccurately drawn. In particular, the nasal opening ap- 

 pears high up on the front of the head and not connected with the mouth by a naso-labial 

 groove. Upon comparing this figure with later stages (Figures 83 and 84, plate VII), the 

 size of the dermal denticles arranged in a V-shaped pattern posterior to the eye appears 

 exaggerated. The dorsal portion of the pelvic fin forms a finger-shaped projection pointing 

 dorsad. This may possibly be an upturned rudimentary myxopterygium. Aside from 



