THE LABILE NATURE OF ORGANIZATION 



33 



Fig. 14. A sea urchin egg 

 is cut in two through the 

 animal - vegetal axis, A-V . 

 Each half is fertilized and 

 each half develops into a nor- 

 mal embryo of half size. 



PLUTEUS 



PLUTEUS 



11). If, however, we cut the egg through the animal-vegetal axis instead 

 of through the equator, we again obtain two halves, and again both halves 

 may be fertilized. But now we find that both halves form complete embryos 

 (Fig. 14). These embryos are half-sized, but they contain all the structures 

 in normal proportion and normal position. As many as four embryos can 

 be obtained from eggs in this way. Each half may be cut again to form 

 quarters. As long as the cut is made parallel to, or runs through, the animal- 

 vegetal axis a whole embryo forms from a fragment of the egg. Therefore 

 we speak of this organization as being somewhat labile or flexible. The egg 

 is sometimes spoken of as a plastic egg. 



Is the egg completely labile? According to our original plan of the 



