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Some Studies on the New Zealand Oysters

Fertilisation and Cleavage

Fertilisation and Cleavage

Fertilisation and cleavage in O. lutaria take place in the inhalent chamber of the parent oyster. The sperm balls rotate rapidly and move through the sea water by means of the lashing sperm tails. After a few minutes in contact with the sea water, the spermatozoa begin to break free from the sperm ball. The heads of the spermatozoa become detached from the central mass of the ball and move outwards and away; the sperm tail is the last part of the spermatozoon to break free. The complete disintegration of a single sperm ball containing approximately 2,000 spermatozoa takes about five minutes. These observations were made by teasing a small portion of a male gonad into a watch glass containing fresh sea water and observing under a microscope.

As fertilisation has never been observed in incubatory oysters, it is assumed that the spermatozoa enter the mantle cavity of a female oyster with the inhalent water current and subsequently fertilise any eggs lying on the gill plates. Three separate attempts at artificial fertilisation of the eggs of O. lutaria were unsuccessful and likewise attempts by other workers to fertilise the eggs of incubatory species of oysters have also been unsuccessful.

However, on one of these three attempts, sperm were noticed clustered around a particular area of each egg. This area corresponds to the flat surface of the egg that had been lying next to the wall of the follicle. This area is thought to correspond to the receptive spot (Text-fig. 2, A).

Before syngamy takes place and possibly just after the sperm nucleus has penetrated the egg, the formation of polar bodies takes place. In some eggs, great clusters of incompletely divided polar bodies were observed while other eggs had only one, two or three polar bodies present. In some instances the polar bodies were noticed to persist through the stages of cleavage and were present on the prototroch of the trochosphere (Text-fig. 2, D).

The two, four, eight celled stages of cleavage were not observed in O. lutaria; the earliest stage observed was the blastula. The result of the early stages of cleavage is that the oosperm becomes differentiated into a megamere (deutomere) surrounded by a large number of micromeres (blastomeres). Finally the micromeres increase to such an extent that they arch over and partly surround the megamere so that the gastrula is formed by epiboly (Text-fig. 2, B and C).