SPAWNING AND SETTING OF OLYMPIA OYSTERS 
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in this respect. It is true, in general, that the young embryos show as pure white in 
the branchial chamber adjacent to the gills, and that older larvae in the well-advanced 
straight-hinge stage appear as a dark-gray or hluish-hlack mass. Yet one may not 
judge accurately the stage of development by estimating the depth of color. The 
Figure 20. — Developmental stages of larvae of Ostrea lurida. From Hori (1933). 1, sperm ball; 2, ovum; 3, ovum with polar body; 
4, first cell division; 5, second cell division; 6, morula; 7, blastula; 8, gastrula; 9, early straight-hinge veliger; 10, veliger larva at 
time of discharge from brood chamber; 10-15, shells of free-swimming larvae of various sizes; 16, full grown larva of setting size; 
17, young spat with growth of new shell. A, anus; A. M., anterior adductor muscle; A. P., posterior adductor muscle; C. S., 
crystalline style; D, digestive diverticula; F, foot; G, gill; Int., intestine; L. sh., larval shell; L. V., left valve; M, mouth; O, 
oesophagus; P. S., pigment spot; R, rectum; R. F., rudiment of foot; R. M., retractor muscle; R. V,, right valve; Sh., shell; 
5. Sh., spat shell; St., stomach; T, teeth; V, velum. 
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