LARVAL SHARKS 217 
an exceedingly slow one; Pristiurus emerges from the 
egg in about nine months, Scyllium in about seven. And 
in consequence of the large amount of yolk stored in 
the yolk sac, the young shark, as in Fig. 289, has fully 
acquired its adult outward characters by the time the yolk 
is exhausted and its sac absorbed. 
In Fig. 284 is figured a stage in the development of 
Pristiurus which may be regarded as either embryonic 
or larval; the form of the larva is well established ; gill 
clefts, muscle-plates, mouth, and sense organs are present ; 
but, on the other hand, unpaired fin and anus are lacking. 
There is shown the abrupt constriction, characteristic of 
Elasmobranchs, which separates the animal from the yolk 
sac, —a construction which in later stages becomes narrow 
and tubular. The relatively larger size of the yolk sac 
in later stages is, of course, the result of the bulkier elabo- 
ration of the yolk material. 
The youngest stage (Fig. 284) shows prominently the 
great enlargement of the anterior end of the embryo, a 
marked cephalic flexure, large optic capsule, and irregular 
gill slits of graded sizes; a tubular tail end, bulbous at 
the terminal, where the neurenteric canal occurs; as yet 
the nasal pits are in close proximity to the mouth. In the 
next stage (Fig. 285), the elongated trunk has its unpaired 
fin, the neurenteric canal disappearing; the beginnings 
of the pectoral fins are noticeable; gill clefts are of more 
uniform size; and the anal region is indicated. In the 
stage of Fig. 286, further advances are seen in the con- 
_ stricting off of the unpaired fins, the appearance of the 
ventral and the continued growth of the pectoral fins; 
in the reduced foremost gill slit (spiracle); in the jaw 
region, and, in fact, in the entire shaping of the head; 
in the appearance of the lateral line. In the ventral head 
