726 NITROGEN METABOLISM AND GROWTH 9 



tion by urease. Control of the pH of the medium to permit (or prevent) urease 

 activity allows correlated measurements of urea production. 



Protozoa have contributed information of another sort, namely, toward the 

 processes related to events in the individual's growth prior to division. These 

 have been made possible by a technique for obtaining dry weight of a protozoan 

 under water (Zeuthen, 1948) and by providing conditions which promote 

 simultaneous divisions of a protozoan culture (Scherbaum and Zeuthen, 1954; 

 Mazia and Prescott, 1954). Mazia (1956) summarizes certain aspects of the 

 individual cell growth: (j) daughter cells are equivalent, (2) growth begins at 

 once at the maximal rate, and (j) deceleration of growth occurs when each of the 

 daughters has doubled its original mass, and as a corollary of this, growth does 

 not occur during division, and {4) there is a time lag, following growth cessation 

 and prior to actual division, amounting to about four hours in a 24-h. cycle. 

 Using the colorimetric technique of Lowry et al. (1951), Prescott demonstrated 

 the virtual identity of the increase in protein with increase in dry weight. By 

 causing an unequal division of a parent, daughter cells of different weight are 

 produced; Prescott observed that, in such an instance, the cells did not simply 

 double their starting weights, but that the two unequal cells arrived at the same 

 end-point, sviggesting a genetic control of size limitation. Further experiments of 

 amputation of cytoplasm demonstrate that (j) division can be postponed in- 

 definitely (Hartman, 1928) and (2) that the amputation may be performed so 

 late in the life cycle of the cell that division is inevitable. 



Several notions have been proposed to explain the triggering of cell division : 

 (7) increase of cytoplasm to the point that a single nucleus cannot serve it, 

 (2) completion of duplication of the genetic endowment of the cell, (j) ratio of 

 mass to surface area, and {4) increase in CO 2 content, perhaps related to the 

 last postulate. It would seem from the experiments above that there is a triggering 

 mechanism, and since this is related to the lag period after accumulation of 

 definitive mass, that the genetic theory is now the most plausible. Prescott (1957) 

 provides a thoughtful review of these problems. 



Mazia and Dan (1952) succeeded in isolating the mitotic apparatus in 

 quantity from sea urchin eggs. Their analyses indicate (j) that it is made up of 

 one kind of protein, part of which is combined with RNA, (2) that it constitutes 

 about 12% of the total cellular protein (more than could be derived from the 

 nucleus alone). Speculations with regard to the method of organization of the 

 mitotic spindle and its activities after it has been formed are less conclusive. 



From the foregoing discussion it is apparent that the role of proteins as energy 

 source may be more important in the sea urchin egg than in amphibia. Pertinent 

 information on the chick is wanting and certainly no more than the barest 

 information is available on the mammal in this regard. There is evidence, cited 

 above, that synthetic activity is high during early development of the sea urchin; 

 that this may also be the case with amphibia is clear from the analyses of Kutsky 

 et al. (1953). They describe a decrease in aspartic acid, glycine, histidine and 

 threonine up to the midblastula period, and an increase in glutamic acid during 

 this period which they feel "may be linked to the subsequent formation of specific 

 proteins of the induction or neuralization process". Similarly, Clayton's (1953) 



