The Embryology of Heterodontus japonicus 745 



lar cleft is closed except for a small dorsal portion. For the first time, in this series, we see 

 rudimentary gill'filaments projecting from gill'clefts — the first, second and third, in this 

 embryo. Due to differential growth of associated parts, the eye is now situated very close 

 to the dorsal end of the mandibular arch where a swelling indicates the anlage of the 

 future maxillary process. The position of the otic vesicle is no longer indicated in surface 

 views. There is a shallow olfactory pit. The hind'gut is fairly well defined. 



Figure 32, plate II, portrays in surface view an embryo with about 85 myomeres. One 

 notes that the yolk stalk is more slender, and that there are five gill-clefts. Gill'filaments 

 project from all five gill'clefts, but there are none from the spiracular cleft. The rudiment 

 of the maxillary process is more prominent than in the preceding stage. 



Figure 33, plate III, represents a surface view of an embryo with at least 88 myomeres 

 (those near the tip of the tail are indistinct). This embryo shows a decided advance over 

 the preceding one. To be sure, no new structures have emerged save a single gill-filament 

 protruding from the spiracular cleft; but all the embryonic structures mentioned in the 

 preceding drawings are present in a more advanced stage of development. The myomeres 

 show a higher degree of differentiation. The rudiments of the fins — pectoral, pelvic, 

 first and second dorsals, anal, and caudal — are recognisable at a glance, though all the 

 median fin rudiments appear to be connected by a fin fold. Both pectoral and pelvic fin 

 rudiments are very broad at the base, but they are evidently not connected by a fin fold. 

 One sees, more clearly than in the two preceding figures, the contour of the brain. The 

 yolk stalk is more slender, as one would expect in this stage. There seems to be some 

 injury to the cardiac region. 



Figure 34, plate III, pictures in surface view an embryo somewhat older than the 

 one shown in the preceding figure, but evidently drawn at a lower magnification. The 

 myomeres are not visible. All the fin rudiments are now discrete: i.e., not connected by 

 a fin fold. Gill-filaments are more numerous, and some are decidedly larger. The spiracu- 

 lar gill-cleft reveals four or five short gill-filaments; this cleft is now somewhat farther 

 from the first and nearer the eye. As compared with the preceding figure, there is 

 a remarkable enlargement of a region of the brain comprising the mesencephalon (mid- 

 brain) and metencephalon (anterior division of the hindbrain, containing the cerebellum). 

 The olfactory pit is larger and deeper. The maxillary process of the mandibular arch is 

 no longer clearly defined in surface views. It extends beneath the posterior rim of the 

 optic cup. 



An embryo in Dean's collection, slightly older than the one represented in Figure 34, 

 plate III, is about 38 mm. (1.5 inches) long. Its external gill-filaments are slightly longer 

 than those represented in this figure. 



Figure 35, plate III, shows almost maximal development of the external gill-filaments. 

 The spiracular cleft is closer to the eye and directly posterior to it. The mouth opening, 

 bordered by rudimentary labial folds, is now recognizable. The pectoral fin is quite 

 large, and the caudal fin is very long. The dorsal fins are taller, and narrower at their 



