60 THE OYSTER. 



opening, the anus, is formed. The tract which con- 

 nects the anus with the stomach lengthens and forms 

 the intestine, and, soon after, the sides of the stomach 

 become folded off to form the two halves of the liver, 

 and various muscular fibres now make their appear- 

 ance within the body. 



Such is the history of the oyster-embryo. Its prac- 

 tical utility is in the fact that while the American oyster 

 lays a vast number of eggs, they are exposed to dangers 

 so constant and innumerable, that under ordinary con- 

 ditions few ever come to life, or at any rate succeed in 

 living long enough to anchor themselves and take on 

 the protection of shells. This is only another example 

 of a fact well known to naturalists. The number of 

 eggs laid, or even of individuals born, has very little 

 to do with the abundance of a species, which is de- 

 termined, mainly, by the external conditions to which 

 it is exposed. 



LIFE OF THE YOUNG OYSTER. The young American 

 oyster leads a peculiarly precarious life, since it is 

 first thrown out an unfertilized egg, so that the chance 

 that it will immediately meet with a male cell must be 

 very slight ; yet if it does not it will perish, for the sea- 

 water soon destroys unimpregnated eggs. Having 

 by good chance become fertilized by meeting a male 

 cell, the next period of great danger is the short time 

 during which the embryos swarm at the surface of the 

 water. They are so perfectly defenseless, and so 

 crowded together close to the surface, that a small fish, 

 swimming along with open mouth, might easily swal- 



