EXPERIMENTAL ANALYSIS 



that embryonic cells in tissue culture cannot enter mitosis in the absence 

 of oxygen. Laser^^^ found that cultures could grow under these cir- 

 cumstances for several days, but did not specifically examine them for 

 mitoses, while Ephrussi et aliP^^ found that cells would migrate, but 

 would not complete division at an oxygen tension lower than 7 mm of 

 mercury. Harvard and Kendal^i^ found that although mitosis could 

 occur in cultures at oxygen tensions much lower than this, reducible 

 substances in the medium deprived of oxygen would finally bring the 

 oxidation-reduction potential of the culture to a value below the 

 threshold for growth. Embryonic cells in culture can enter mitosis, in 

 the presence of inhibitors of some stages of glycolysis, such as malonate 

 (PoMERAT and Willmer;216 Hughes^^i) and fluoracetate (Allsopp and 

 Fell;^^' Hughes^"^) and respiratory poisons such as carbon mon- 

 oxide (Figure 66) and hydrocyanic acid (Pomerat and Willmer;^^® 

 HuGHEs^^^). The apparent indifference of the dividing cell in cultures 



Figure 67 Respiration of eggs of the sea 

 urchin Psammechinus miliaris from the 4th 

 to the 8th cleavage. Oxygen uptake 

 measured as rate of movement of the mano- 

 meter of the Cartesian diver micro-respiro- 

 meter. From Zeuthen^^* [By courtesy, Biol. 

 Bull.). Cleavage of the egg occurs near the 

 minima of the curve. 



of embryonic tissues to these inhibitors may well be related to the fact 

 that their environment contains an adequate supply of glucose. Pace^^^ 

 has shown that the effect of cyanide on respiration in Paramoecium 

 depends on the extent to which the respiratory mechanism is saturated 

 with carbohydrate. 



Thus the available evidence would not warrant a general statement 

 that cells can only enter mitosis under aerobic conditions. It is probable 

 that the reaction of cells in division to experimental agents is super- 

 imposed on the general metabolic pattern of the particular tissue which 

 may vary at different stages of development. Zeuthen^^^ found that the 

 respiratory rhythm of cell division in echinoderm eggs increases in 

 amplitude after the sixth cleavage (Figure 67). The energy require- 

 ments of mitosis at different stages in development may thus be variable. 



Adenosine and the adenylic acids at appropriate concentrations 

 prevent the entry of cells into prophase in the whole animal (Bullough 

 and Green^^°) and in tissue cultures (Hughes--^) where, however, these 

 substances exert no effect at any other point in mitosis.* Runnstrom 



* Berrian and Dornfeld^^'* find that the number of mitotic figures in cultivated rat 

 ovaries is significantly reduced by cytidylic, guanylic, and yeast adenylic acids. 



189 



