II UTILIZATION OF PROTEIN PRECURSORS 497 



In the concluding section an attempt will be made to discuss the main problem 

 of embryonic differentiation in the light of concepts and results developed recently 

 in some related fields of biology such as microbiology and immunology. 



II. THE UTILIZATION OF PROTEIN PRECURSORS IN THE 

 DIFFERENTIATING CELL 



(a) Precursors larger than amino acids 



In the study of the nature of protein precursors and of their utilization in em- 

 bryonic cells two different, but not necessarily mutually exclusive hypotheses have 

 been explored. One line of research has been designed to test the possibility that 

 precursors of a molecular size larger than amino acids, as large perhaps as protein 

 molecules, give rise to the protein molecules of the differentiating embryonic cell. 

 Schechtman (1955) who, with his associates, contributed much experimental 

 work to this problem, has recently summarized evidence that proteins can actually 

 be transferred through cellular membranes and taken up as undegraded mole- 

 cules into embryonic cells ^ Indeed, uptake of immunoproteins and serum proteins 

 tagged with fluorescent dyes has been shown to occur in the rapidly growing 

 cells of neoplastic tissues (Busch and Greene, 1955). However, uptake of, at 

 least small protein molecules into mature cells is also indicated by observations 

 of experimentally produced effects of ribonuclease on various intracellular struc- 

 tures (Kaufmann and Das, 1955) and by the demonstration of the uptake of 

 homologous antibodies into adult tissues (Humphrey and McFarlane, 1954). 

 Indications of an incorporation of protein precursors larger than amino acids were 

 also obtained in interesting work with cells in vitro (Francis and Winnick, 1953 

 and Gerarde and Jones, 1953). An effect of the intact proteins in cell growth in 

 vitro is indicated by the gradual adaptation of cells to heterologous media (Lang- 

 man, 1953a, b) and the reactions to antibodies (Nace, 1955). 



Actual utilization of large molecular cell components taken up into embryonic 

 cells was demonstrated by the observation that organ implants accelerate specifi- 

 cally the growth of the homologous organs in the host embryo. Weiss (1947) 

 postulated that large molecular material released from these transplants is taken 

 up specifically by the cells of the homologous host organ and acts catalytically as a 

 "template" for the protein forming apparatus of these cells. As in so inany other 

 instances in embryological research, the failure to perform a quantitative analysis 

 led to this erroneous assumption. By labelling the proteins of the donor tissue 

 with methionine-^^S, Ebert could compare quantitatively the amount of label lost 

 from the donor with the amount of label taken up specifically by the homologous 

 host organ (Ebert, 1954a). He found that within the range of experimental accu- 

 racy the observed increase in the host organ could be accounted for by the amount 

 lost from the donor tissue. These experiments did not support the assumption of a 

 catalytic role of the transferred high molecular material which was apparently 



1 An increased permeability of embryonic cells to large molecules is also indicated by the 

 distribution of inulin in muscle tissue often to twelve day chick embryos (Herrmann, White 

 and Cooper, 1957). 



Literalure p. 53g 



