1883.] on Oysters and the Ouster Question. 347 



upon it. And tlie first problem is, Where are these ova and sperma- 

 tozoa formed ? Does each oyster produce both, or are they formed in 

 distinct oysters ? This is, in fact, the vexed question of the sexes of 

 the oyster, which has been the subject of so much discussion, and for 

 which the answer is gradually shaping itself, thanks mainly to the 

 recent labours of Mobius and Hoek. 



I have already stated that if the surface of the trunk of a full- 

 grown oyster is examined carefully with a lens, or even without one, 

 a curious ramified and more or less reticulated whitish marking, which 

 is very obvious in the breeding season, is observable beneath the thin 

 integument. By appropriate methods of investigation it is easily 

 determined that this marking is produced by the ramifications of a 

 tubular organ, — the reproductive gland — the trunk of which debouches 

 into a cavity common to it and the renal organs, which again, it will 

 be recollected, communicates by a narrow slit with the supra-branchial 

 chamber (Fig. 2 (A), ug). The trunk of the gland, on each side, 

 passes upwards and backwards, in front of and above the adductor 

 and muscle, and gives off a multitude of branches, some of which cross 

 the middle line and become inextricably united with those of the other 

 side, while others form a network beneath the skin which covers the 

 stomach and the liver. From this network, blind ofishoots are given 

 off perpendicularly inwards, and extend for a variable depth into the 

 interior of the body. The whole extent of the walls of the tubes of 

 this reproductive gland is lined by nucleated cells, and it is by the 

 metamorphoses of these cells that the ova, on the one hand, and the 

 spermatozoa, on the other, are produced. 



During the breeding season, an examination of the adult oysters 

 on an oyster-bed shows that the number of individuals the repro- 

 ductive glands of which contain hardly anything but ova is about 

 equal to that of the individuals in which the reproductive gland 

 contains hardly, any thing but spermatozoa. I say " hardly anything" 

 because competent observers have affirmed, that careful search will 

 always reveal a few spermatozoa in the former, and a few ova in the 

 latter. Whether this be so or not, there can be no doubt that, 

 practically, oysters, while actually breeding, are either males or 

 females. 



When the ova or spermatozoa are ripe, they flow out of the repro- 

 ductive gland into the surrounding water. The spermatozoa are 

 carried away by the exhalent currents of the oyster in which they are 

 developed, and are doubtless drawm in by the inhalent currents of 

 adjacent oysters, the eggs of which they fertilise. And, as the eggs 

 already exhibit the first of that series of changes which lead to the 

 formation of the larva, when they leave the reproductive gland, it 

 would appear that they must undergo fertilisation while still within 

 that organ. 



The eggs which pass into the supra-branchial chamber must also 

 be di'iven out by the exhalent current ; but it would seem that, when 

 they reach the hinder edge of the branchial partition, they come 



