EFFECTS OF AGING UPON GERM CELLS. 397 



physiologically poor condition, and when fertilized by n-hour- 

 old sperm gave o per cent, cleavage. These same eggs tested by 

 freshly liberated sperm gave the same results, namely o per cent. 



Hence the results obtained by fertilizing fresh eggs by old sperm is 

 determined in largest part by the physiologic condition of the eggs. 

 The sperm plays but a minor role. One must be cautious, however, 

 in any given experiment, in inter pretng the results. In experiment 

 3, for example, the lo-hour-old sperm gave only 10 per cent, 

 cleavage with fresh eggs. This low cleavage might have been 

 due to the physiologically poor condition of the sperm or of the 

 eggs. Actual tests showed that the "fresh eggs were in poor con- 

 dition and not the old sperm. 



If the sperm deterioration is not reflected, or only to a very 

 slight degree reflected, in a reduction in cleavage how does it 

 happen that in all experiments when aged eggs were fertilized by 

 aged sperm the cleavage was far less than in either of the other two 

 crosses. 



This result is probably due to a summation of effects, that due 

 to the deteriorated condition of the eggs, and that due to the 

 probable greater difficulty of aged sperm to initiate mitosis and 

 cleavage. In the final analysis both are reducible to the con- 

 dition of the egg. 



Arbacia. 



Further light was thrown upon these questions by the experi- 

 ments with Arbacia. 



Experiment 12, Table VI., is illustrative of the effect of aging 

 upon the sperm. In this experiment after the germ cells no 

 longer cleaved, the old sperm was tested against fresh eggs of 4 

 different females, over a long range of intervals. When the sperm 

 were 28 hours old, the eggs of the 4 females averaged 85 per cent, 

 cleavage (90 minutes after fertilization). When the sperm 

 were 45^ hours old 81 per cent, of the fresh eggs of 4 other 

 females cleaved; when 73 hours old 86 per cent.; when 77 hours 

 old ''very many" (exact count not taken}; when 95 hours old 59 per 

 cent, of the eggs cleaved. At this last observation the different 

 females gave 29, 59, 72 and 78 per cent, cleavage. How much 

 longer the sperm could have fertilized freshly liberated eggs was 

 not determined. How much of the reduction (when the sperm 



