BIRTH, GROWTH AND REPRODUCTION OF OYSTERS. 113 



examined, the reproductive organ will be found no longer 

 to contain ova, but abundant developing and fully formed 

 spermatozoa. After producing eggs, in fact, the female 

 oyster changes its sex and becomes male. (/) 



The conclusion, first advocated by M. Davaine many 

 years ago, that the same individual oyster is alternately 

 male and female, is therefore unquestionably correct. 

 What has yet to be made out is the period of recurrence of 

 this extraordinary alternation of sexes. Do oysters change 

 their sexes once or more than once in a season ? Until 

 this point is ascertained, all calculations as to the propor- 

 tionate number of oysters which breed during a season, 

 based on the observation of the proportion of those which 

 at any given time contain fry, are obviously unsafe. If, for 

 example, the alternation took place once a month, not 

 more than half the oysters might at any time contain fry, 

 and yet, in four months, every oyster might have spatted 

 twice. 



In the case of the Portuguese and the American 

 oysters, in which both the reproductive products pass at 

 once into the water, and no incubation takes place, arti- 

 ficial fecundation is easily effected. The embryos develop 

 normally, pass through their changes within the egg, and 



(/) According to Davaine, the oysters are said to produce only 

 male sexual products towards the end of the first year, and it is only 

 later, from the third year onwards, that they become females and 

 produce ova. 



Mcebius, on the contrary, asserts that the sperm is the later 

 formed, and not until after the pregnant beast has got rid of her eggs. 

 The reproduction takes place especially in the months of June and July, 

 at which time, in spite of their extraordinary fertility, the oysters must 

 not be gathered. "Text Book of Zoology," p. 25. (Claus and 

 Sedgwick.) 



