of the Fishery Board for Scotland, 
315 
the trunk and the rectum. It is small in fig. 17, but in fig. 18 both the 
vesicle and its duct are seen in close approximation to the rectum, the 
duct opening immediately b(diind the anus. Figs. 20 and 21 also show 
urinary duct and urinary vesicle, though the duct is not always apparent 
when larvae of this age are viewed as a whole. 
The yolk-sac, viewed by some as part of the ovum and by others 
as an appendix of the ovum, is considerably reduced by the time 
of the hatching of the embryo. The yolk-mass during the absorption 
of the yolk by the larva is gradually retracted from the v/ail of the 
sac, and the surface of the yolk-mass exhibits stellate pigment spots 
which are fewer in number as the yolk-mass decreases. The appearances 
indicated during this change are illustrated by figs. 15-21, y. 
On the tenth day the eml)ryonic or marginal fin (w./., fig. 12) appears 
along both the dorsal and ventral aspect of the posterior part of the trunk. 
The dorsal portion of the fin on the eleventh day (fig. 13) is about double 
the length of the ventral marginal fin, and during the subsequent life of 
the embrj'^o a marked increase takes place both in the length and in the 
breadth of their marginal fins. After the birth of the larva the marginal 
fin becomes very broad, extending from the head along the dorsal surface 
of the body round to the ventral surface as far as the anus. The caudal 
fin-rays (c./.) radiating outwards from the tail are seen during the same 
period (figs. 18 and 21). The pectoral fins ip.f.) arise as lateral outgrowths 
from the body at about the same time as the marginal or embryonic fin is 
distinct (fig. 1 2). They have a broad base when they arise, but the limb 
subsequently becomes proportionally broader. The clavicular part {cl.) of 
the pectoral girdle is visible from the stage represented by fig. 17 onwards. 
EXPLANATION OF REFERENCES. 
a. anus. 
au. auditory organ. 
aur. auricle. 
hp. blastopore. 
hr. blastodermic rim. 
ch. choroidal fissure. I 
ct. caudal fin. 
cl. clavicle. 
f. h. fore-brain, m.h. mid-brain, Ti.h. hind- 
brain. 
g. gut, m.g. mid-gut, h.g. hind-gut. 
g.J). gall-bladder. 
ht. heart. 
ir. iris. 
K.v. Kupffer's vesicle. 
I. lens. 
Ir. liver, 
m. mouth. 
mf. marginal fin. 
mn. mandible. 
my. myotomes. 
I nc. notochord. 
ne. neurochord. 
ol. olfactory organ. 
op. optic lobe, optic vesicle, and eye. 
p.f. pectoral fin. 
jm. pineal gland. 
pr. protovertebrse. 
St. stomach. 
u.v. urinary vesicle. 
ven. ventricle. 
y. yolk and yolk-sac. 
EXPLANATION OF PLATES VII.-IX. 
Fig. 1. Blastoderm, 6 hours old, dorsal view x 45. 
Fig. 2. Blastoderm, 26 hours old, dorsal viewx 40. 
Fig. 3. Blastoderm, 43 hours old, dorsal view x 40. 
Fig. 4. Blastoderm, 67 hours old, showing embryonic rim and embryo dorsal 
view X 40. 
Fig. 5. Blastoderm, 79| hours old, dorsal view x 40. 
Fig. 6. Embryo, 4 days 2 hours old, sho-wing brain and body, dorsal view x 40. 
Fig. 7. Embryo 4 days 7 hours old, showing divisions of brain and proximal periphery 
of blastopore, dorsal view x 40. 
Fig. 8. Embryo, 5 days 64 hours old, showing protovertebrse, Kuppfer's vesicle, and 
elongated blastopore, dorsal view x 40. 
Fig. 9. Embryo, 5 days 19^ hours old, showing sense-organs, dorsal view x 40. 
