FALL OF THE SPAT 113 



spermatozoa being formed and fecundating these ova — the 

 ' black-sick ' stage. In this latter view the oyster is first 

 female and then male, and is, both morphologically and physio- 

 logically, hermaphrodite. The old view, that ' black-sick ' 

 oysters are the male, and ' white-sick ' the female, is therefore 

 quite incorrect. 



The ova, in their earliest stage, consist of minute oval 

 clusters of globules floating in a transparent mucus. They 

 pass from the ovary into the gills and folds of the mantle, and 

 are probably fecundated within the excretory ducts of the ovary, 

 before arriving in the mantle chamber. In this stage the oyster 

 is termed ' white-sick.' In about a fortnight, as the course of 

 development proceeds, the fertilised ova become ciliated at one 

 end (the so-called veliger stage, p. 131), and soon pigment 

 appears in various parts of the embryos, giving them a darker 

 colour, which varies from grayish to blue, and thus the white- 

 sick oyster becomes ' black-sick.' When the black spat emerge, 

 they are still furnished with cilia for their free-swimming life. 

 This is of very short duration, for unless the embryo finds some 

 suitable ground on which to affix itself within forty-eight hours, 

 it perishes. As the spat escapes from the parent oyster, which 

 slightly opens its valves and blows the spat out in jets, it 

 resembles a thick cloud in the water, and is carried about at the 

 mercy of wind and tide. April to August are the usual spawning 

 months, warm weather being apparently an absolute necessity 

 to secure the adhering of the spat. A temperature of 65° to 72° 

 F. seems requisite for their proper deposit. Thus on a fine, 

 warm day, with little wind or tide running, the spat will fall 

 near the parents and be safely secured, while in cold, blustering 

 weather it will certainly be carried off to a distance, and prob- 

 ably be altogether lost. The number of young produced by 

 each individual has been variously estimated at from 300,000 to 

 60,000,000. Either extreme seems enormous, but it must be 

 remembered that besides climatal dangers, hosts of enemies — 

 other Mollusca, fish, and Crustacea — beset the opening career 

 of the young oyster. 



As soon as the spat has safely 'fallen,' it adheres to some 

 solid object, and loses the cilia which were necessary for its 

 swimming life. It begins to grow rapidly, increasing from 

 about 2V ii^ch in diameter to about the size of a threepenny 



VOL. Ill I 



