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PACIFIC SCIENCE, Vol. XV, April 1961 
0 hours 
0 hours 45 min. 
12 hours 45 min. 
14 hours 
I hour 15 min. 
18 hours 20 min. 
4 hours 
6 hours 
25 hours 
27 hours 
Fig. 9- Developing eggs of Acanthurus triostegus sandvicensis. Egg diameter, 0.67 mm. 
the yolk plug is visible as a small clear area near 
the oil globule. 
By 15 hr. 30 min. the body of the embryo is 
beginning to take form and elongate over the 
yolk. At 18 hr. 20 min. 9 somites can be counted 
and the optic vesicles are prominent. At 21 
hr. 45 min. 12 somites are present. At 25 hr. 
19 or 20 somites are visible; the lens of the eye 
is just starting to form; the auditory vesicle is 
present and the statoliths are just forming; 24 
tiny melanophores can be seen in lateral view 
on the body; twitching movements are common; 
the heart contains blood although it has not yet 
started to beat. 
Hatching occurs at the age of about 26 hr. 
At 27 hr. the larva is nearly 1.7 mm. in length 
and has 22 somites; the heart is still quiescent. 
At 31 hr. the heart was observed to beat; oc- 
casional short random swimming movements oc- 
cur by rapid vibration of the posterior half of 
the body. 
At the age of 42 hr. the amount of yolk is 
reduced to about half of what is present in the 
31 -hr. larva. As the yolk is used up, the larvae 
show a progressive tendency to sink. Larvae of 
39 hr. still float head down at the surface, but 
42. 5 -hr. fish have begun to settle. The rate of 
settling of 1 larva at this age was measured at 
1.2 cm. per min. The average rate of 3 44.5-hr. 
larvae was 1.67 cm. per min., and the average 
rate of 4 47-hr. larvae was 1.88 cm. per min. 
One 71-hr. larva sank at the rate of 8.2 cm. per 
min. 
At 42.5 hr. some larvae were already settling 
out at the bottom of the aquarium. Most, how- 
ever, maintained their level by swimming move- 
ments. 300 such movements were observed in 
an aquarium which contained several hundred 
larvae. Of these movements, 227 resulted in the 
larvae being in a higher position in the tank; the 
rest were sideways or downward. Movements 
which consisted only of a twitch were not 
