OYSTERS 



sk- 



FIG. 33. Development of the egg of the common oyster, after fer- 

 tilisation within the tubular passage of the reproductive sacs. 

 A, surface view. B, section through a very early stage the 

 separate cells or protoplasmic corpuscles which have resulted 

 from the dividing up of the fertilised single egg-cell are seen ; 

 bl, in-pushing to form the gut ; sk t in-pushing to form the rudi- 

 ment of the double shell. C and D, the same a few hours 

 later. The mouth, m, is now seen. E, still later stage surface 

 view: a ring of cilia has appeared. F, the young free-swimming 

 oyster nearly ready to leave its mother's protection, who is now 

 laden with such young, and is said to be " white-sick." The 

 top of the head, tp, is now well marked and surrounded by a 

 ring of lashing cilia. The outline of the right-side shell is seen, 

 and the foot,//, between the mouth, m, and the arius, a. The 

 stomach, st, and the intestine, e, show by transparency. 



