The Embryology of Chlamydoselachus 595 



An Embryo Measuring 20 Millimeters 



The next drawing called for on the "List" is of this size. It bears the notation ''Emb. 

 A 20 mm." The original drawing is marked xlO+ but it measures 222 mm. The 

 drawing represents the embryo (in lateral view) as pulling forward on the yolk sac 

 (Figure 18, plate II). There is fair correspondence between this embryonic Chlamydo' 

 selachus and Scammon's 20.6'mm. Squalus (his Fig. 28, pi. III). Compared with the 15.5' 

 mm. embryonic frilled shark, the forebrain of the 20'mm. specimen is more prominent, the 

 midbrain has become swollen laterally and is separated from the hindbrain by a con' 

 striction of its lateral and (morphologically) ventral surface. The dorsal line of this 

 embryo, in contrast with the younger ones, runs almost straight to what is evidently the 

 anlage of the dorsal fin — located just behind the vertical through the cloaca. The tail 

 bends down sharply without, however, any upturned point as in the preceding stage. 

 It is more like that in the 11.5'mm. specimen (Figure 15, plate II). 



Returning to the head region, attention is called to the changed olfactory anlage and 

 to the much enlarged eye. The mouth is somewhat less widely open than in the preceding 

 stage. The gill-arch region is more prominent than ever. As in the preceding stage, some 

 of the arches are in their lower halves sharply angled forward. There are eight distinct 

 gill-arches with seven gill-clefts. The first visceral (the mandibular) arch forms the upper 

 and lower jaws (palatoquadrate and Meckel's cartilage respectively). In the stage shown, 

 with wide-open mouth, the jaws are open at an angle of about 90°. The ridge immediately 

 behind the eye is probably not a branchial arch but a part of the cranium; likewise the 

 two short ridges dorsal to the one just mentioned are presumably also eminences of 

 the cranium. 



The arch immediately behind the mandibular arch is the second visceral or the hyoid 

 arch. Its dorsal half, lying immediately behind the protuberance of the skull previously 

 mentioned, will give rise to the hyomandibular cartilage. This, in the adult, articulates 

 with Meckel's cartilage at the angle of the jaw, thus helping to support the jaw. The 

 first branchial cleft appears to be closed ventrally. Its dorsal portion, lying immediately in 

 front of the dorsal segment of the hyoid arch, will become the spiracle. For the position 

 and relations of the spiracular canal in the adult, see Smith (1937, Text-figs. 82 and 84). 

 The other visceral arches (the gill-arches of the adult) are quite regular in form save for 

 the crumpling already mentioned. 



The bulge on the ventral surface of the embryo, immediately in front of the yolk 

 stalk, is caused by the heart. The pectoral fin has become much broader and looks to be 

 almost functional in a rudimentary fashion. The pelvic fin now shows clearly. The 

 wart-like cloacal eminence is but little larger than that shown in the 15.5-mm. embryo. 

 In front of it, the pelvic fin is clearly outlined. Back of it is the rudiment of the anal fin. 

 The tail ends in a point sharply hooked downward. The somites are far advanced and now 

 have the perfected zig zag shape faintly foreshadowed in the preceding stage — 15.5 mm. 

 (Figure 17, plate II). 



Scammon's Fig. 28, pi. II, of his 20.6-mm. Squalus is further developed than Dean's 

 20-mm. Chlamydoselachus. The dogfish has all the fins, the pectoral being better de- 



