468 
BULLETIN OF THE BUREAU OF FISHERIES 
be concluded that an individual seldom, if ever, spawns as a female while carrying a 
brood of larvae. 
Records of the larvae taken in such collections were arranged in tabular form 
(fig. 19) to show the percentage of adults beeping larvae of different stages of develop- 
ment. By connecting the values from date to date and stage to stage, the age of each 
group is made clear. Division of the developmental process into 10 stages is largely 
arbitrary but at the same time convenient. While the em- 
bryonic stages, up through the trochopliore, are well defined, 
the only significant difference between straight-hinge larvae 
of different ages is in size. Measurements with an ocular 
micrometer were made of the larvae in each sample and while 
there is necessarily some uncertainty as to exact size, because 
of the variation among the larvae themselves, this is not 
great enough to be confusing. 
The example reproduced (fig. 19) is only one of many 
which are at hand but there is little difference between them. 
In the first place it will be noted that a total period of 9 to 
1 1 days is required for development from eggs to the largest 
straight-hinge larvae. The general embryology has been 
well described by Stafford (1914) and is essentially the same 
as in 0. virginica, so that it is not necessary to describe it 
here. To illustrate some of the important stages there is 
reproduced in figure 20 a series of drawings from Hori (1933), 
an original copy of which Professor Hori kindly prepared 
for the writer. The figures are drawn accurately to scale so 
that they may be employed for the identification of larvae 
of the species. 
When they are discharged from the gonad the eggs 
are 100 to 105m in diameter, as stated by Stafford (1914) 
and Hori (1933). Development proceeds much more slowly 
than in the case of oviparous species. On the day after the 
eggs are discharged into the brood chamber they have be- 
come blastulae. At the age of 2 days they are usually in 
the gastrula stage, and 1 day later they have developed 
the swimming organ, or prototroch, and are actively swim- 
ming trochopore larvae. Usually on the fourth day the 
small valves may be seen developing on the dorso-lateral 
surfaces as a pair of clearly defined structures about 30 to 
40^ long. This may be called the first conchiferous stage, 
and in figures 15 to 18 they are considered as such and in- 
cluded in the shaded portions of the columns. On the fifth 
day the valves have become complete and enclose the 
larvae entirely except when they are swimming with the velum protruding. In 0. 
virginica, as stated by Stafford (1913), “The age at which swimming begins may be 
considered to be about 5 hours, reckoned from fertilization * * As indi- 
cated above, the early embryology proceeds much more slowly in the viviparous 0. 
lurida, requiring between 3 and 4 days to reach the swimming stage. 
Larvae carried in the brood chamber are commonly spoken of as being either 
white or black. The expressions, “white-sick” and “black-sick” are frequently used 
Figure 19. — Graph showing percent- 
age of adult oysters bearing broods 
of larvae of each of 10 stages, as fol- 
lows: 0, eggs, or early segmenta- 
tion; 1, blastulae; 2, gastrulae; 3, 
trochophores; 4, first conchiferous 
larvae with incomplete valves; 5 to 
10, straight n-hinge veliger larvae 
classified according to approximate 
length of valves; 5, 110-120^; 6, 
120-130;i; 7, 130U40 m; 8, 140-155m; 9, 
155-170^; 10, 170-185ju- The per- 
centage values of larvae of definite 
size groups are connected to indicate 
rate of development. See text. 
