348 Professor Huxley [May 11, 



within the influence of the inhalent current and are thereby swept 

 back into the infra-branchial chamber. Here they accumulate, and 

 becoming imbedded in a viscid albuminous matter, secreted by the 

 parent, constitute the " white " fry (Fig. 2 (B) ). 



From the nature of the case, this account of what takes place is 

 not the result of direct observation ; but it seems to be by far the 

 most probable explanation of the facts which can be observed. In an 

 oyster which contains white fry, in fact, the reproductive gland is 

 flaccid, and contains nothing, or hardly anything, but a few unex- 

 pelled ova. 



The case is different, however, with oysters the eggs of which 

 have been laid so long that they have passed into the condition of 

 " black spat." Here many, or, as I have recently found in one case, 

 the great majority, of the tubes of the gland contain developing sper- 

 matozoa, while only a few exhibit ova. And Dr. Hoek has recently 

 made the important observation that, if an oyster which contains fry 

 is kept for a fortnight in an aquarium by itself and then examined, 

 the reproductive organ w411 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, artificial fecundation is easily effected. The 

 embryos develop normally, pass through their changes within the 

 egg, and their locomotive stage, into the condition of fixed oysters 

 rapidly, when confined in properly arranged aquaria. It is probable, 

 therefore, that artificial breeding will sooner or later be practised on 

 a great scale with these oysters. In the case of our own oysters, 

 artificial propagation by the methods practised in the case of the 

 Portuguese and American forms, which involve the destruction of both 

 parents, is obviously out of the question, unless some substitute can 

 be found for the process of incubation, during which it is probable 

 that the young oysters receive, not merely shelter but nourishment, 

 from the parent. But a careful study of the conditions under which 

 our oysters breed freely, will no doubt enable oyster cultivators to 



