470 
BULLETIN OF THE BUREAU OF FISHERIES 
pure whiteness of the eggs and young embryos slowly changes toward the gray after 
development of the valves. As the larvae grow older the mass becomes darker and 
darker, while the valves develop and pigment forms in the tissues. Frequently the 
largest larvae appear as black with a somewhat bluish tint, but it often happens that 
larvae of the largest size ever found in the brood chamber are only a medium gray. 
After 5 days of development in the brood chamber the larval valves become com- 
plete and the larvae are in the straight-hinge stage, so called because the dorsal 
border of the valves is straight, in contrast with the later pronounced umbo in the 
hinge region. The further developmental stages are arbitrarily arranged according 
to length of the larval valves, as measured under the microscope. 
These results appear not to be out of harmony with those of Stafford (1914) save 
that in the present case development is somewhat more rapid. His estimate of a 
period as long as 16J4 days may be correct for the locality in which he worked, or it 
may be due to his method of analysis. His data on water temperature, though in- 
complete, appear not to differ greatly from those taken in the present instance. 
Coe’s (1931a) estimate of a period of about 10 to 12 days required for development 
within the branchial chamber is more in accord with the records described above, 
which indicate a period of about 10 days, on the average. 
In his work on 0. edulis Orton (1926) estimated the gestation period, and stated 
(p. 219): 
An analysis of the spawning oysters into those with young embyros and those with mainly 
shelled larvae brings out the fact statistically that oyster larvae under natural conditions are retained 
in the mantle cavity a period of only 1 to 1)4 weeks from the date of their extrusion as fertilized 
eggs from the parent. 
In a later paper (1936) he wrote: 
The white-sick stage is thus normally of about 3 to 3J4 days duration, the grey-shelled stage 
about 1 34 to 2 days or less, and the black-sick stage of variable duration, probably 4 days or less. 
It seems probable that the oyster larva becomes fully developed normally in the sea in a period of 
6 or 7 days and is expelled at an age between 7 and 10 days. 
Apparently his samples were not taken with sufficient frequency to permit 
analysis in the manner described above. Nevertheless it is probable that the period 
of larval development within the maternal brood chambers is not greatly different 
in the two species. 
Stafford used the word “swarming” to designate the final release of larvae from 
the maternal brood chamber, in contrast to the original spawning whereby the eggs 
are released from the gonad. It may be considered, in viviparous species, that swarm- 
ing is the delayed completion of the spawning process which in oviparous oysters, as 
described by Nelson (1928c) and Galtsoff (1930b-1932), is accomplished at once by 
means of rhythmic contractions of the adductor muscle. Whether discharge of the 
larvae is accomplished in the same manner is not known, but it has been observed that 
during abortions the embryos are forcibly ejected by means of shell movements. 
After discharge the larvae live and grow as free-swimming organisms for a period 
of approximately 1 month. The largest larvae found in the brood chambers are 
180 to 185^ lo n g> as described by Stafford (1914) and Hori (1933). The smallest 
straight-hinge larvae, 5 days after fertilization of the eggs, are about 110 to 120m 
long. Growth in length of the valves within the brood chamber proceeds at a rate 
of about 12 m per day. At the time of setting the larvae are almost constant in size, 
of a length very close to 320 m- As is described in the following section, at least 30 
days elapse between the time of discharge of the first larvae and the time when the 
