110 The Nature of Biological Diversity 



species. Control eggs, injected with the saline solution alone, devel- 

 oped normally except for occasional abnormalities due to physical 

 injuries produced by the injection procedure. However, eggs injected 

 with several of the nuclear fractions exhibited a consistent develop- 

 mental arrest at the beginning of gastrulation. Cell division stopped 

 in these arrested blastulas, but otherwise they appeared normal. Since 



7 vtW 



i 



i 



1. FERTILIZATION 



2. INJECTION 



3. ARRESTED BLASTULA 



4. DISSOCIATION OF CELLS 



5. ACTIVATION 



6. ENUCLEATION 



7. NUCLEAR IMPLANTATION 



ARRESTED BLASTULA 

 (FIRST TRANSPLANT 

 GENERATION) 



SERIAL TRANSPLANTATION 

 OF NUCLEI 



ARRESTED BLASTULA 

 (SECOND TRANSPLANT 

 GENERATION ) 



FIG. 7. Diagram of the procedure used in testing the developmental effects of in- 

 jected protein fractions extracted from adult frog-liver nuclei. Nuclei derived from 

 injected eggs were serially transplanted for seven generations, but each generation 

 arrested at the beginning of gastrulation. 



