AMPHIBIANS 



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by the various authors as plasmones, cyto- 

 genes, plasmagenes or gene products, which 

 become likewise capable of reduplication 

 and mutation, and which are directly re- 

 sponsible for the physiological and morpho- 

 logical properties of the various types of 

 differentiating cells (Wright, '41, '45; Dar- 

 lington, '44; Haddow, '44; and others). 



It has been sviggested by several authors 

 that the hypothetical plasmagenes are com- 

 parable to, or located within, the basophilic 

 granules (mitochondria, microsomes) re- 

 ferred to above, which in turn share some 

 properties with the virus (for further ref- 

 erences see Brachet, '45, '50). Needham ('42) 

 compares homoiogenetic induction with the 

 mechanism of virus infection. Many workers 

 are inclined to think that some tumors orig- 

 inate from the transformation of normal 

 microsome-like entities into virus particles. 

 This idea seems to be supported by the find- 

 ings of Claude ('40, '41), who isolated from 

 normal chick tissues small nucleoprotein 

 particles which could not be distinguished 

 chemically from the infective virus which 

 renders these tissues malignant. Thanks to 

 the investigations of Claude ('46), Brachet 

 and collaborators ('40, '42, '44) and many 

 others, it has been established that cyto- 

 plasmic granules of a comparable composi- 

 tion, though of various sizes, are present in 

 at least the majority of embryonic and 

 adult tissues. Many of the essential enzymes 

 have been found to be localized in the larger 

 granules (mitochondria), and it has been 

 suggested that the alleged capability of re- 

 duplication of these granules is due to their 

 richness in ribonucleic acid. 



It will be recalled that Brachet attaches 

 great importance to the function of the 

 nucleoprotein granules in morphogenesis. Al- 

 though we were reluctant to accept the data 

 now available as evidence to show that any 

 of the described constituents of these gran- 

 ules are specifically engaged in neural in- 

 duction, one can hardly doubt that in view 

 of their being the carriers of so many impor- 

 tant compounds, the granules play an essen- 

 tial role not only in the metabolic 

 specification of adult tissues but also in the 

 developmental elaboration of the different 

 tissues. 



On the basis of his studies on enzymatic 

 adaptation of yeasts, Spiegelman ('48) has 

 arrived at similar ideas and has proposed 

 a concrete scheme which he considers to be 

 applicable to embryonic differentiation. The 

 chief merit of this scheme consists in its 

 attempt to translate the notions of potential- 



ity, competence and induction into the more 

 tangible terms of enzymology. As is pointed 

 out, it is the tiniqueness of the enzyme 

 patterns more than anything else that dis- 

 tinguishes the tissues from each other. Ac- 

 cording to Spiegelman, genes determine 

 merely the potentiality of enzyme formation, 

 but whether or not a particular enzyme is 

 actually formed in the cytoplasm depends 

 upon other factors, of which the substrate 

 is obviously one. It is assumed that en- 

 zyme formation is governed by autosynthetic 

 reduplication and that in this process 

 the various enzyme-forming precursors or 

 "plasmagenes" compete with each other 

 for nitrogenous compounds. An externally 

 applied specific substrate is presumed to 

 combine with one of the inactive and rather 

 unstable precursors to form a specific plasma- 

 gene-enzyme system which thereafter can 

 reproduce itself faithfully. Thus the sub- 

 strate enhances the continued production and 

 accumulation of one type of enzyme at the 

 expense of other potential enzyme-precursors 

 which through competition are more or less 

 crowded out. However, owing to cytoplasmic 

 patterns of distribution and to certain "sym- 

 biotic" relations and ecological interactions 

 within the population of the competing en- 

 tities, there remains room for the co-exist- 

 ence of different enzymes within a cell. 

 Just the same, "the fate of any given cell 

 dvuring morphogenesis will be determined by 

 the outcome of the competitive interactions 

 amongst the initial plasmagene population." 

 (Spiegelman, '48; see also Steinbach and 

 Moog, Section III, Chap. 2 of this book.) 



If plasmagene population means "com- 

 petence," and activating substrate "induc- 

 tor," then the above considerations may serve 

 as a model for interpreting the relations 

 between these phenomena. Competence may 

 then be ascribed to the stage-specific pre- 

 ponderance of certain enzymatic or morpho- 

 genetic precursors which can combine se- 

 lectively with certain inductive agents. By 

 doing so, the newly formed self-reproducing 

 compounds would gain an advantage over 

 the other competing precursors, and the 

 cell would progressively narrow down its 

 competence to become eventually unipotent. 

 But since competence changes with time 

 independently of external stimulation, either 

 progressive transformation of the plasma- 

 genes themselves, or their selective elimina- 

 tion through cell-bound competition must 

 be assumed. The prevailing irreversibility 

 and "canalization" of differentiation (Wad- 

 dington, '48) would result from the "survi- 



