OLE MAAL0E AND KARL G. LARK 



K. E. Cooper. Have you tried cell wall stains ? Many bacteria have transverse septa 

 which divide them into multicellular organisms. 



L. Rinaldini. Has any electron-microscopy been done on these cells during the decline 

 in lysogenesis, to see if the septum develops gradually in each cell or if it appears 

 abruptly and the gradual decline is obtained by more and more cells dividing? 



0. Maaloe. Studies by means of sectioning and electron-microscopy are in progress. 

 At the moment we can say only that there is no indication of a morphologically well- 

 defined septum being formed before cell division begins. 



E. Ambrose. Is there any change in the sensitivity of the bacterial nuclei to phage at 

 various stages of the division cycle ? With larger cells the nuclear membrane dis- 

 appears during metaphase. It seems likely that the phage nucleoprotein, which is of 

 high molecular weight, could combine more easily with the nuclear material in the 

 absence of a membrane. 



O. Maaloe. The available data seem to indicate that there is no particularly sensitive 

 period during the division cycle. With a high multiplicity of infection as many as 

 80 per cent, of the bacteria become lysogenic. It is not likely that within the short 

 time during which the decision for or against lysogenization is made, the majority of 

 the cells would pass into any specified phase of the division cycle. Also, if a particularly 

 sensitive phase existed, we might expect the peak in the frequency curve to rise by 

 more than a factor of two. As to the second point, there is some evidence that in the 

 bacteria we have studied there is no well-defined nuclear membrane. 



E. jV. Willmer. I should like to call attention to some observations, on tissue cultures 

 of chick fibroblasts, which seem to be somewhat parallel to those reported by Dr. 

 Maaloe. These cells, when grown in flasks in a plasma medium, cease to show cell 

 divisions in the outgrowth after about sixty hours; but, if they are then treated with 

 embryo juice for about an hour, they can be caused to divide again for a limited 

 time; such divisions start after about ten hours and cease after about twenty-four 

 hours. Repetition of the dose of embryo juice before the cells start to divide is without 

 effect, but if the second dose is delayed until the cells are dividing, then a second 

 crop of divisions results in due course: thus, the activating agents, possibly with 

 nucleoproteins among them, can only gain limited access to the cells and the latter 

 can become 'saturated' until the situation is changed by the occurrence of the 

 division process. (Jacoby, F., Trowell, O. H. and Willmer, E. N. (1937). J. exp. Biol. 



14 ' 255 ") 



Secondly when chick cultures are cooled to a temperature below io° C. divisions 



temporarily cease, but when the cultures are returned to 37 C. there is a compen- 

 satory excess of cell divisions. (Spear, F. G. (1928). Arch. exp. ^ellforsch. 7, 484.) 



By combining these two sets of observations a method for obtaining more nearly 

 synchronous mitoses in chick fibroblasts would appear to emerge as a possibility. 

 0. Maaloe. I am very pleased that a point like this has been raised, because I feel 

 that in our paper it is not emphasized strongly enough that temperature shift is 

 but one means out of a great many which may be tested for their suitability to pro- 

 duce synchronous behaviour in cell populations. 



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