rERTILIZED EGG 



Fk;. 3. A sea urchin egg in the process of fertilization. The fertilized egg 

 divides once, and two cells result. Each of the two cells divides, and the four 

 resulting cells are shown, 1, 2, 3, 4. The four-cell stage is then placed in sea 

 water from which calcium has been removed. The Ca-free sea water dissolves a 

 protein which holds the four cells together and a mild shaking separates them. 

 The four cells are separated and continue to develop into sea urchin larvae, which 

 are called plutei. Each of the four larvae has all the structures of the normal larva 

 but is only one fourth normal size. Presumably each pluteus would grow and 

 transform into a normal adult sea urchin. How does the original egg develop 

 into a single individual when several individuals may be obtained from one egg? 



