76 Ce 



Differentiation 



the embryo. Weissmann realized that the embryo would ultimately be- 

 come an adult animal, that it would produce eggs or sperm, and that these 

 would necessarily have to contain all the genetic elements to make the 

 next generation of embryos. He made provision for this by adding the 

 qualification that during cleavage a few cells would inherit a complete set 

 of genetic elements (see Fig. 38) and that these would become the 

 gametes of the future adult. 



In order to test this hypothesis, biologists permitted egg cells to 

 cleave into 2, 4, 8, 16, or more cells and then removed some of them. If 

 all the cells had inherited a complete set of instructions for making an 

 embryo, the loss of a few cells would not be expected to matter. In con- 

 trast, if most of the cells received an incomplete set of instructions, the 

 removal of some cells would remove certain instructions that the other 

 cells lacked and an incomplete embryo would result. Experiments of this 

 kind have tended to group embryos into two categories. If we permit a 

 fertilized egg to cleave into two blastomeres and then separate them, 

 each will, it is true, produce a complete embryo. However, if the egg hap- 

 pens to be from a clam, starfish, sea urchin, or comb jelly, this capacity 

 disappears rapidly. That is, once the egg has cleaved into 8 or 16 or 32 



Fig. 38. A schematic illustration of Weissmann's hypothesis (the segrega- 

 tion of genetic elements). 



