I I 2 OYSTERS, AND ALL ABOUT THEM. 



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

 the reproductive gland into the surrounding water. The 

 spermatozoa are carried away by the exhalant currents of 

 the oyster in which they are developed, and are doubtless 

 drawn in by the inhalant 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 fertilization 

 while still within that organ. 



The eggs which pass into the supra-branchial chamber 

 must also be driven out by the exhalant current ; but it 

 would seem that, when they reach the hinder edge of the 

 branchial partition, they come within the influence of the 

 inhalant current, and are thereby swept back into the infra- 

 branchial chamber. Here they accumulate, and becoming 

 embedded in a viscid albuminous matter, secreted by the 

 parent, constitute the " white" fry. 



From the nature of the case, this account of what 

 takes place is not the result of direct observation ; but it 

 seems by far the most probable explanation of the facts 

 which can be observed. In an oyster which contains 

 white fry, in fact, the reproductive gland is flaccid, and 

 contains nothing, or hardly anything, but a few unexpelled 

 ova. The case is different, however, with oysters the eggs 

 of which have been laid so long that they have passed into 

 the condition of "black spat." Here many, or, as I have 

 recently found in one case, the great majority of the tubes 

 of the gland contain developing spermatozoa, while only a 

 few exhibit ova. And Dr. Hoek has recently made the 

 important observation that, if an oyster which contains fry 

 is kept for a fortnight in an aquarium by itself, and then 



