34 NOTES, ETC., OX THE AUSTRALIAN ROCK OYSTER. 



fully formed. An oyster eaten durinf>- the later phases of the 

 breeding season, appears to be full of sand or grit, this being 

 due to the presence of the many millions of minute embryonic 

 shells. By oyster dealers at home, two distinct spawning 

 conditions of the oyster are recognised : the one, when the 

 embryos contained within the mantle chambers of the parent 

 are white and colourless, being devoid of shells, is designated 

 the " white sickness "; and the later stage when, the shells being 

 formed, a grey or blackish tint is imparted to the entire mass, 

 is known as the " black sickness." The close or spawning season 

 of the ordinary f^uropean oyster. Ostrca I'Jnlh, extends through- 

 out the summer, from j\Iay to September, and is popularly defined 

 as being represented by those months in which the letter " r " is 

 absent. 



The fecundation of the ova of the European oyster 

 necessarily takes place within the mantle cavity or brood- 

 chambers of the female, the fertilising fluid or milt of the male 

 being discharged into the water, and from thence it is absorbed 

 and brought into contact with the mature ova by the ciliary 

 currents that exercise the ordinary respiratory and food-purveying 

 functions in the female mollusc. This plan of propagation was 

 until within recent year? supposed to apply to all descriptions of 

 oysters. Investigations associated with the reproductive pheno- 

 mena of the American commercial oyster, <htrca riniinedua 

 failed, however, to discover any trace of the brood or spat within 

 the mantle cavities of the breeding oyster, and it was ultimately 

 demonstrated by Dr. Brooks (18.S0), that both the ova and milt 

 were simultaneously discharged into the water in their mature 

 condition, and fertilisation l)eing there effected, the entire 

 development of the embryo took place independently of the 

 j)arent. Such being the case the artificial propagation of the 

 species by the commingling in sea water of the matured sexual 

 elements was considered feasible, and was successfully accom- 

 plished by the above-named authority. In the case of the typical 

 European oyster, Ostna i<htJis, such a method of artificial 

 propagation is not possible, chiefly on account of the fact that 

 the embryos are matured within the brood-chambers of the 

 parent in a fluid medium, containing a large proportion of 

 albuminous matter that cannot be artificially produced. Follow- 



