THE OYSTER. 55 



up into a layer of cells, and in a sectional view the 

 embryo is seen to be made up of two layers of cells ; 

 an upper layer of small transparent cells, which are to 

 form the outer wall of the body, and which have been 

 formed by the division of the spherules which occupy 

 the upper end of the egg, and a lower layer of much 

 larger cells which are to become the walls of the stom- 

 ach, and which have been formed by the division of the 

 large spherule. 



This layer is seen in the section to be pushed in a 

 little toward the upper layer, so that the lower surface 

 of the disk-shaped embryo is not flat, but very slightly 

 concave. This concavity is destined to grow deeper 

 until its edges almost meet, and it is the rudimentary 

 digestive cavity. Fig. 7. A very short time after this 

 stage has been reached, and usually within from two to 

 four hours after the eggs were fertilized, the embryo 

 undergoes a great change of shape. Plate IV, Fig. 8. 



A circular tuft of long hairs, or cilia, now makes 

 its appearance at what is thus marked as the anterior 

 end of the body, and as soon as these hairs are formed 

 they begin to swing backward and forward in such a 

 way as to constitute a swimming organ, which rows the 

 little animal up from the bottom to the surface of 

 the water, where it swims around very actively by the 

 aid of its cilia. This stage of development, which is 

 of short duration, is of great importance in rearing the 

 young oysters, for it is the time when they can best be 

 siphoned off into a separate vessel and freed from the 

 danger of being killed by the decay of any eggs which 



