36 NOTES, ETC., ON THE AUSTRALIAN ROCK OYSTER. 



may in consequence be easily effected and yields a most interest- 

 ing and instructive embryological study. The method of ))ro- 

 cedure successfully adopted in accomplishing such artificial 

 propagation, and the more conspicuous metamorphoses throu!:!;h 

 which the embryo passes before assuming the parent form, may 

 be described as follows : — 



The aid of a microscope with a magnifying power of about 

 200 diameters is, in the first instance, indispensable for securing 

 the most satisfactory results. On opening a number of oysters, 

 the cream-coloured fat-like mass near the hinge or joint of the 

 bivalve shell represents the seat of the reproductive elements. 

 Inserting a fine spatula into the midst of this mass a small 

 portion may be abstracted, and spread out in a drop of sea water 

 or the natural juices of the mollusc on an ordinary glass slip. 

 Placed under the microscope, the ova or germs of the female 

 oyster will be at once recognised by their ovate or pyriform con- 

 tour, the separate ova having an average diameter of the 3-Joth 

 part of an inch. The male elements or spermatozoa when 

 abstracted and similarly treated, present a widely different aspect. 

 Its separate elements are so diminutive as to appear as minute 

 granules only under the same magnification, and a considerably 

 higher amplification is requisite to illustrate their individual 

 structure. This is then shoAvn to consist of a minute bulbous 

 head and an exceedingly slender flexible hair-like tail, the pro- 

 portions between the two being much the same as that of the 

 head and shank of an ordinary pin. After a little experience 

 it will be found easy to distinguish the comparatively coarse 

 granular ova from the cloudy masses of spermatozoa, when 

 placed on the glass slip, with the aid only of an ordinary pocket 

 lens or even the unassisted vision. The assistance of the micro- 

 scope is, however, desirable to insure the most favourable results, 

 and is altogether indispensable for tracing the further develop- 

 ment of the embryos. In many instances it will be found, what 

 can be recognised only with the microscope, that the ova or 

 spermatozoa, are not sufficiently matured, or in the case of oysters 

 purchased in the market ha^ e become deteriorated by isolation 

 from their native element for too long a period to permit of per- 

 fect fertilisation. The conditions being satisfactory, the ova 

 under the microscope should present a clean and evenly rounded 

 outline, while the vitality of the spermatozoa should be manifested 



