I GROWTH OF THE GERM-CELLS 19 



(Infusoria) changes, periods of rapid division alternating with 

 periods of depression, or slow division, and it would seem from 

 the figures given by Woodruff for the mean rate of division 

 for a succession of five-day periods, that at any moment the 

 rate is roughly proportional to the number of divisions that 

 have taken place since the last period of depression, and to the 

 number that will occur before the next period of depression is 

 reached : also that the rate is at a maximum when half the 

 divisions have occurred. In other words, the rate of division 

 of the Protozoon changes in the same way as the rate of 

 growth of the Metazoon. It follows, of course, that the rate 

 of growth of the former, as expressed by the average incre- 

 ment of the total weight (or other dimension) of all the indi- 

 viduals produced by division of one, cannot obey the same 

 law. At the same time, some figures published by Popoff for 

 the growth of a single individual of an Infusorian (Frontonia) 

 between two divisions, indicate that the growth-rate rises to 

 a maximum in the middle of the process, and may therefore 

 possibly depend at any moment on the amount grown already, 

 and the amount still to be grown. If this should prove after 

 further investigation to be really the case, then the growth of 

 the individual Protozoon would be comparable with the growth 

 of the individual Metazoon ; in both cases the rate at any 

 moment would depend on the two factors named. A further 

 problem remains. While we can understand that the rate 

 should depend upon the amount of living and growing sub- 

 stance, x, it is not so easy to see what meaning is to be 

 attached to the other factor, A x. In the case of the chemical 

 reaction this is the amount of substance left unchanged. 

 Robertson has suggested that in the organism this is cytoplasm, 

 or some ingredient in the cytoplasm from which nuclein can 

 be synthesized, the synthesis of nuclein being the supposed 

 autocatalytic unimolecular reaction upon which growth 

 depends. The idea is based upon Loeb's assumption that there 

 is such a synthesis during segmentation in the (Sea-urchin) 

 ovum. This is, however, erroneous, since Masing has shown that 

 there is as much nucleic acid in the unsegmeafeed as in the fully 

 segmented egg. In the former it lies in the cytoplasm, into 



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