magnesium starved embryos were immobile 

 indicated that immobile magnesium deficient 

 embryos (Shumway stages 21-23, swimming 

 tadpole) made fewer ribosomes than did control 

 embryos of a comparable age (Fig. 3). 



The apparent decrease in R-RNA synthesis 

 in magnesium starved embryos could be ex- 

 plained by one or more of the following hypoth- 

 eses: 1) Ribosomes were made at the normal 

 rate in the magnesium starved embryos but 

 ribosomal turnover was accelerated; 2) The 



rate of synthesis of R-RNA was slower in 

 magnesium starved embryos than in controls; 

 3) There was a failure to assemble all newly 

 made R-RNA into ribosomes. 



The following experiment was performed 

 to determine the stability of ribosomes in 

 immobilized magnesium starved embryos. Em- 

 bryos were grown in 10% Holtfreter's solution 

 and at Shumway state 20-21 were labeled with 

 ^'*C02 and then incubated for 20 hours in non- 

 radioactive 10% Holtfreter's solution. At the end 



(a) 24 HOUR CONTROL 

 (DORSAL LIP) 



(b) 24 HOUR HYBRID 

 (DORSAL LIP) 



30 10 



TUBE NUMBER 



E 

 o 



CM 



d 

 d 



0.9 



0.6 



0.3 



(C) 48 HOUR CONTROL 

 (EARLY NEURULA) 



(d) 48 HOUR HYBRID 

 (DORSAL LIP) 



20,000 

 15,000 

 10,000 

 5,000 



f 900 

 600 



-\ 300 

 



TUBE NUMBER 



Fig. 2. 



Sedimentation patterns of RNA extracted from ^^ P-labeled whole control and hybrid em- 

 bryos: a) 50 twenty-four hour control embryos, b) 50 twenty-four hour hybrid embryos, 

 c) 55 forty-eight hour control embryos, d) 55 forty-eight hour hybrid embryos. Sibling 

 embryos were used in this experiment. 



38 



