538 Phylum Mollusca 



ulated by the eggs of various mollusks (Venus, Mya, Mytilus), thy- 

 roidin, theelin, extract of thymus, various sugars, starch and other 

 organic compounds (Galtsoff, 1935). 



Naturally fertilized eggs may be obtained by the following method. 

 Ripe females should be kept out of water for several hours or overnight, 

 then placed in a glass tank, 15-20 liter capacity, in water of about 25°- 

 2 7 C. As soon as the oyster opens the shell, which occurs almost 

 immediately provided the organism was conditioned by being kept out 

 of water, about 1 cc. of sperm suspension, made of 0.5 gm. of testis in 

 50 cc. of seawater is injected between the shells. Spawning begins after 

 a latent period varying from 6 to 30 minutes and may last nearly an 

 hour. The female must be removed as soon as a sufficient number of 

 eggs is obtained. Since the eggs are fertilized in the water as they are dis- 

 charged by the female, various cleavage stages become mixed together. 



For embryological and cytological work it is preferable to have all 

 the eggs fertilized simultaneously. A piece of ovary cut from the gonad 

 is gently shaken into a finger bowl or larger jar. For insemination one 

 or two cc. of sperm suspension is added, the water stirred and the jar set 

 aside. All large pieces of tissues should be carefully removed. 



Fertilization takes place immediately and the eggs settle to the bottom. 

 Both polar bodies are formed within half an hour after fertilization. 

 The rate of development is dependent on temperature. At 23°-2 5° C. a 

 free-swimming stage is reached within 4%-5 hours. The larvae rise to 

 the surface, forming easily noticeable vertical streaks in the water. 

 They should be collected by means of a wide-mouthed pipette, trans- 

 ferred into fresh seawater and left undisturbed until the next day. A 

 method of rearing them to metamorphosis is described in the following 

 article by H. F. Prytherch. 



The spawning reaction of the Japanese oyster, 0. gigas, is similar to 

 that of 0. virginka with the exception that under laboratory conditions 

 higher temperature is required. In the experiments carried out by the 

 author at Woods Hole, the females of this species were stimulated to 

 spawn by the sperm of either species and by raising the temperature 

 of the water to 30 C. 



Cross fertilization between the two species occurs very readily (Galt- 

 soff and Smith, 1932). (See Editor's Note on p. 546.) 



Bibliography 

 Brooks, W. K. 1880. Development of the American oyster. Studies from Johns 



Hopkins Univ. Biol. Lab. 4:1. 

 Galtsoff, P. S. 1930. The role of chemical stimulation in the spawning reactions 

 of Ostrea virginica and Ostrea gigas. Proc. Nat. Acad. Sci. 16:555. 



1930a. The fecundity of the oyster. Science 7 2: 97. 



1932. Spawning reactions of the three species of oysters. J. Wash. Acad. 



Sci. 22:65. 



