46 Our Food Mollusks 



which represents a section cut through the center of the 

 mass. The outer cells, darkly shaded in the drawing, 

 will form the wall of the body, the inner ones the di- 

 gestive tract. The reference-letters ^ / indicate a pit or 

 cavity, now wide open to the surface, but later nearly 

 closed, which becomes the stomach. The reduced open- 

 ing is the primitive mouth. 



Up to this time, the shape of the embryo has ap- 

 proached that of a sphere, but it now changes, assum- 

 ing a form like that represented by VIII. The outlines of 

 surface cells are shown here, but in subsequent figures 

 they are omitted. At one end of the body the surface 

 cells throw out some protoplasmic hairs, or cilia, which 

 begin a lashing or rowing movement so violent that 

 the embryo is raised from the bottom, where it has 

 been lying, and swims upward to the surface of the 

 water. 



From two to four hours have now elapsed since sper- 

 matozoon and ovum united. As in the case of many 

 other animals, the rate of segmentation depends on tem- 

 perature. When this is lower than the optimum, devel- 

 opment is retarded, and may be stopped altogether. The 

 swimming embryos of the oyster are greatly affected by 

 cold, and a heavy shower or cold wind coming when they 

 are gathered at the surface, may kill them all. This ap- 

 parently often happens on the north Atlantic coast, and 

 probably accounts for the failure of the " set of spat " 

 which sometimes occurs in restricted localities, or even 

 generally, in certain years. Such a failure is almost un- 

 known in the Gulf of Mexico. On the other hand, the 

 reproduction of the eastern oyster is almost entirely 

 prevented in the cold waters of the Pacific, where it is 

 planted and is able to grow. 



