The Embryology of Chlamydoselachus 611 



marked. This is to be seen all the way from head to the dorsal fin. Pectoral and (especial- 

 ly) pelvic fins show development. The dorsal fin is now clearly seen in the drawing and 

 the upper lobe of the caudal stands out on the thicker vertebral part of the tail. 



Seen from the Side. — More marked are the differences in the lateral views of the two 

 embryos — the older (66 mm.) shown in Figure 39, plate IV, and the younger (54 mm.) in 

 Figure 33 on plate III. The head of the older specimen is smaller and is curiously round' 

 ed. The eye is larger, the nasal groove has moved forward. The mouth is closed and the 

 lower jaw is plainly longer. The spiracle is placed in about the mid'lateral line whereas it 

 is above it in the 54'mm. fish. The first gill-cover has a ragged or frilled edge and seems 

 retracted — as it is in the 54-mm. fish and (particularly) in the 46-mm. specimen (Figure 

 29, plate III). The gill-filaments in general are smaller, fewer and not so far protruded. 

 The "back-of-the-neck" hollow seen in Fig. 33 has here become a great ''sway-back" de- 

 pression, giving the idea of a definite necl{ between head and body. The body is more 

 humped than that of the 54-mm. embryo. The paired fins show growth. Dorsal and 

 anal are larger and better differentiated. The tail bends gracefully downward. The 

 ventral lobe is here sharply separated from the well-developed anal fin, unlike the close 

 approximation seen in the 54-mm. fish. This is the earliest embryo showing the tail fin 

 in approximately the adult condition. 



Dean's 66-mm. embryo contrasts strongly with Carman's 64-mm. specimen. It has 

 the top of head high and rounded. In the neck region it has a long "sway-back", the body 

 is decidedly arched, and the tail behind the dorsal-anal vertical bends down strongly. 

 The dorsal region of Carman's fish (Text-figure 29a) is nearly straight, having at most 

 very flat curves. Even more difference is to be found in the shape of the gill-flaps. Those 

 of Dean's fish are convex posteriorly, save the first which has a frilled edge standing nearly 

 vertical. For the rest — mouths, spiracles, fins, and tail-tips are very like each other. 



In Ventral Aspect. — Seen from below (Figure 40, plate IV), the 66-mm. fish 

 shows considerable development compared with the 54-mm. specimen (Figure 34, plate 

 III). The head in front of the mouth is greatly shortened and more blunt. This has 

 brought nares and eyes closer to the front of the head. The long mouth begins plainly to 

 foreshadow that of the adult. The inner surface of the upper jaw is serrate, probably due 

 to the presence of rudimentary teeth which have not yet erupted. Both upper and lower 

 jaws are narrower in the transverse and longer in the sagittal plane — more like the adult. 

 The head, back of the angle of the jaws, shows a marked constriction. The sensory- 

 canal system is clearly portrayed. The gill-covers are still pretty widely distended, but 

 with their outer edges bent toward the rear. The gill-filaments still protrude but less than 

 in the preceding stage. The confluent first gill-covers form a convex U over the isthmus. 

 Pectoral fins show little difference from those of the 54-mm. fish, but the pelvics are much 

 further developed. The cloaca has become a longitudinal slit and on either side of its 

 hinder end the abdominal pores make their first appearance. As in the 54-mm. embryo, so 

 here, in the mid-ventral line is the rudiment of the tropeic folds. The anal fin and the 

 ventral lobe of the caudal are fairly distinct. 



