530 MOLECULAR MECHANISMS OF DIFFERENTIATION 5 



14th day. After the iGthday RNA accumulation per DNA was found to occur at 

 the same rate as in controls but the rate of protein accumulation was only 52% of 

 the accumulation rate of the controls. In pitviitary deficiency RNA and protein 

 accumulation is affected to the same extent. However, DNA accumulation 

 continues for a longer period of time than in control embryos (Love and Konigs- 

 berg, 1958). 



Two points of interest emerged from these experiments. The first is the same 

 restriction of responsiveness of muscle tissue to hormones which Moog compares 

 with embryonic competence (absence of growth stimulation in muscle by thyroxin 

 before the 14th day). However, while in embryology the designation of competence 

 is as yet merely a name for an observed phenomenon, several concrete alternatives 

 can be proposed as experimentally verifiable mechanisms for the changes in 

 reactivity of muscle tissue to thyroid and pituitary hormones. Secondly, in devel- 

 oping muscle tissue the disturbances in protein formation are associated with 

 alterations in RNA production and indicate profound effects on the PFS. The 

 delayed accumulation of both RNA and protein per cell may actually indicate 

 that protein synthesis is impaired by a failure of RNA formation. 



V. conclusions: the convergence of concepts 



In the present chapter the protein forming system of the cell was made the center 

 of the problem of differentiation during embryonic development. The intention in 

 following this approach was to show that the conceptual and experimental isola- 

 tion, which has plagued embryology hitherto, has ceased to exist. Some of the points 

 of convergence with other fields of biology have been touched upon in preceding 

 sections of this chapter. It remains, then, for this concluding section to summarize the 

 manifold relationships between embryonic differentiation and other fields of biology. 



As pointed out previously, in studying the changes in the amino acid content of 

 embryonic tissues, in measuring the uptake of labelled amino acids into the 

 proteins of the embryos, and by exploring the effects of amino acid analogs, the 

 approaches to the question of precursor utilization in protein formation in em- 

 bryonict issues, in unicellular organisms and in adult tissues of higher animals have 

 found at least common methodological denominators. However, the relatively 

 high concentration of peptides in the tissues of some embryos and the observation 

 of an uptake of presumably high molecular cell constituents into embryonic cells 

 suggests the possibility that proteins or larger protein breakdown products, as 

 such, can be utilized as protein precursors and that embryonic cells may be in some 

 way unique, in this respect. 



Another question concerns the maintenance of the amino acid pool. In experi- 

 ments with yeast Halvorson and Spiegelman (1953) and Halvorson, Fry and 

 Schwemmin (1955) have shown that the amino acid pool can be replenished by 

 the breakdown of cellular proteins and that the amino acids supplied in this way 

 can be utilized for the synthesis of specific proteins such as the induced enzyme 

 galactosidase. Apparently, the proteins which represent the source of the amino 

 acids are unessential for the maintenance of the yeast cell during this period and 



