732 NITROGEN METABOLISM AND GROWTH g 



The finding of Prescott (see above. Protozoa) may also have a bearing on this 

 proposal. He finds that (j) the adult, attained size in Amoeba proteus is constant, 

 and that protein increase corresponds to total growth; (2) during the period of 

 logarithmic increase of the cytoplasm, the nuclear volume increases only slightly, 

 but sharply increases after the attainment of "adult" cytoplasmic volume. 

 Although attainment of final cell volume seems to be inversely correlated with 

 nuclear volume increase, it is apparent that the nucleus in some way governs 

 the attainment of final cytoplasmic size (and protein content). It is possible that 

 synthesis of cytoplasmic protein is governed by RNA content, that cessation of 

 growth occurs by fixation of cytoplasmic RNA to or by protein, and that final 

 impotence for further protein synthesis occurs when all cytoplasmic RNA is so 

 fixed. Renewal of growth activities then could occur only by replenishing cyto- 

 plasmic RNA from the nucleus, as, e.g., following amputation of cytoplasm; the 

 stimulus for migration of new RNA from the nucleus might simply be increased 

 water uptake caused by the disturbance of the cortical area by the operation of 

 amputation. 



Ebert, in a series of reports has attempted to resolve doubts raised in the 

 experiments described above. Preliminary results (Ebert, 1951) indicated from 

 C-A grafts of adult chicken and mouse spleen that the stimulating effect of adult 

 spleen on growth of embryonic spleen was real and class-specific, for mouse spleen 

 elicited no such response in the chick. Certain other tissues of the adult chicken, 

 however, did have stimulatory eflfects, viz., liver and thymus, but the spleen 

 effect began only on the fourteenth day of incubation. He felt that common 

 possession of certain antigen molecules by liver, spleen and thymus might be 

 responsible for their common effect. The minimum number of antigens which 

 appear on the 14th day in the spleen is three (Ebert, 1952). He has not demon- 

 strated that these are responsible for the growth stimulating effect, however. 

 Later (Ebert, 1954) he showed that corresponding to a 4-fold increase in weight 

 of the host spleen, there is a 4-fold increase in nitrogen content. By C-A transplant 

 of tissues from adult chickens and mice previously injected with -^^S labeled 

 methionine, he demonstrated again that mouse tissue components are not taken 

 up selectively by homologous host organs, but chick spleen and kidney components 

 are. By quantitative analysis of the radioactivity when incorporated in host tissue 

 and when administered independently of the host tissue (yolk sac injections of ^^S 

 methionine, coupled with untreated C-A transplant of spleen), Ebert concludes 

 that protein molecules or some portion thereof actually are incorporated into the 

 host, and moreover, in such quantity that the hypothesis of a "template" or 

 catalytic activity of a few molecules cannot be supported. Nor does he believe 

 that whole cells are transferred from graft to host spleen. In a further set of 

 experiments (Ebert, 1955) he reports no inhibition of spleen development when 

 splenic transplants are made to the coelom of 66-72 h. chicks, nor when homo- 

 genates of fresh adult spleen are injected into the coelom. 



Johnson and Leone (1955) observed inhibition of growth both in vivo and in 

 vitro at the primitive streak stage, when the embryos were grown in an environ- 

 ment of antiserum. Selective inhibition of heart differentiation, as well as general 

 inhibition of growth, resulted from treatment of the embryo with anti-actomyosin. 



