THE GENERATIVE ORGANS OF THE OYSTER. 275 



Another question remains, Do the reproductive organs of the oyster 

 produce ova and spermatozoa in regular alternation ? Robin^ on 

 theoretical grounds, and other authors as the result of observation, 

 have asserted that oysters are androgynous hermaphrodites — that is 

 to say, hermaphrodites which function first as fertilizing males, and 

 afterwards as females requiring fertilization. Mobius (Auster und 

 Austernwirthschaft, p. 20) simply states that ova and spermatozoa 

 are not developed at one and the same time in the reproductive organs 

 of the oyster, but successively ; that spermatozoa may be formed 

 soon after the ova are laid, and that probably, in the same season, 

 half the oysters in any locality produce ova only, and the other half 

 spermatozoa only. Dr. Hoek agrees entirely with the last-named 

 author. An oyster which contains ripe spermatozoa in the breeding 

 season almost always shows younger stages of spermatozoa in its 

 reproductive organs, whilst, on the contrary, an oyster temporarily 

 acting as a female, and full of ripe or nearly ripe ova, has all its 

 ova in nearly the same condition ; it follows, therefore, that it is able 

 to produce spermatozoa for a long period, but that all the ova are 

 laid at nearly the same time. This last point can also be demon- 

 strated by the examination of an oyster which has very young spat 

 in its branchiae ; in this case the reproductive organs are void of ova. 

 In the case of oysters which come under the second category on 

 p. 273, one may well suppose that they have acted and will act as 

 males, but as females only in the following year ; nevertheless there 

 is no proof that this must necessarily be the case. But a very simple 

 experiment, which has been several times mentioned, shows that 

 oysters which have acted as females begin immediately to produce 

 spermatozoa, and the latter may very probably serve in the same 

 year for the fertilization of ova produced by other oysters. 



Age at which Oysters become Sexually Mature. — Gerbe has examined 

 a great number of young oysters (425, a year old). Of these 35 

 had spat in their branchiee, 127 had their ovaries full of ova, and 

 189 had spermatozoa. It is doubtful, however, whether the oysters 

 on the cultivated grounds of the East Schelde reproduce themselves 

 when only one year old. Of a number of well -developed oysters 

 which were opened at the end of the first year some appeared as 

 though they would function as males in the following summer, others 

 undoubtedly gave the appearance of being about to develop mature 

 ova in the following year. It is impossible to decide whether these 

 latter oysters must necessarily produce ripe spermatozoa first. 



However, it is not at all impossible that the oysters of the East 

 Schelde may be inferior in this respect to their relatives in a state 

 of nature. If manipulation exercises an unfavorable influence on 

 the number of breeding oysters, it is not at all unlikely that it should 



