KFFKCT OF LACK OF OXYGEX. 



When the division plane has completely formed before total 

 absence of oxygen, it remains, and when oxygen is admitted 

 the cleavage goes on regularly, the egg dividing into two equal 

 cells, then four, etc. The division planes which come in during 

 a scarcity but not total absence of oxygen (i.e., about 15 min. of 

 hydrogen) are often somewhat crinkled and irregular (Fig. 42). 



If the egg is subjected to lack of oxygen just before the second 

 cleavage, when air is admitted, the same peculiar irregular 

 divisions are formed as when exposed to hydrogen during the 

 first metaphase (i.e., just before the first cleavage), followed by 

 fusion and irregular divisions, but resulting in normal blastulae. 

 If subjected to lack of oxygen at any stage between the com- 

 pletion of the first cleavage and the full metaphase of the second, 

 the cleavage goes on normally when air is admitted. 



These experiments show that sea urchin eggs may be deprived 

 of oxygen at any stage, from just before the union of the two 

 pronuclei until the completion of the first cleavage, and will 

 remain in whatever stage they may be in, without further 

 development, if kept without oxygen. Astral rays, whether of 

 the sperm aster or the first cleavage asters at any stage, gradually 

 disappear until the egg becomes quite homogeneous. The first 

 cleavage furrow may start to come in during lack of oxygen, 

 but is later resorbed. When oxygen is again admitted, the 

 astral rays reappear in exactly the same position where they 

 disappeared, the mitotic figure reappears in the same phase, and 

 development proceeds. The reappearing mitotic figure and 

 subsequent cleavage is perfectly normal at any stage except in 

 those eggs which were kept without oxygen at the metaphase or 

 a little later, when the reappearing mitotic figure is peculiar 

 and the cleavages are quite irregular. Even the very abnormal 

 looking eggs resulting from the irregular cleavages, however, 

 right themselves and give rise to perfectly normal larvae. 



In Loeb's ('95) experiments, the eggs were exposed to lack of 

 oxygen at only one stage, i.e., soon after fertilization, and his 

 results are quite in accord with mine for this stage. Demoor 

 ('95) thought that in the cells of Tradescantia, one could make a 

 distinction between protoplasmic life and nuclear life in their 

 relation to oxygen, the latter going on in spite of lack of oxygen 



