56 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



that the food materials of the medium penetrated to the 

 interior of cells in the deeper part of the block in two hours 

 time in sufficient quantity to explain the degree of mito- 

 chondrial growth that took place. It should be mentioned 

 that the medium had the consistency of a rather firm jelly. 



Another property of mitochondria has been revealed by 

 the culture experiments. This concerns the utiUzation of 

 urea by the mitochondria of the fetal rabbit hver. The 

 fact that mitochondria may utihze urea suggests a possible 

 mechanism for destruction of urea in the fetus. J. Whit- 

 ridge Wilhams ('17), investigated urea elimination in 

 pregnancy, and found that it did not deviate appreciably 

 from the normal. E. C. P. Wilhams ('23), Hellmuth ('23), 

 and others, have investigated the blood urea during preg- 

 nancy, and have found a reduction below the normal. 

 This would appear to indicate, in conjunction with the urea 

 culture experiments, that the mitochondria of the fetus not 

 only utiUze the urea produced in the fetal metabohsm, but 

 may even extract it from the mother's blood. If it should 

 be found that mitochondria are the stimulating influence in 

 cell division, as it appears possible, then we, apparently, 

 have a tentative explanation for the rapid growth of the 

 fetus. The mitochondria have a special predelection for 

 urea and grow rapidly in its presence, the cells in which 

 they occur undergo frequent divisions with a consequent 

 rapid enlargement of the fetus as a whole. If this property 

 of mitochondria proves to be true in the fetus, then they 

 must change in regard to the character of the food utiUzed 

 in the adult. That such a change of diet actually occurs 

 has been indicated by the culture experiments. 



Dr. K. S. Chouke, working in this laboratory, is investi- 

 gating the development of the capacity for urea ehmination 

 from the kidneys in rabbits. His experiments to date ap- 

 pear to indicate that urea elimination first begins at birth 

 and is gradually increased after birth. 



