Genes: Nature and Mode of Action - 531 



mechanism of genie control. A particular 

 bacterial cell, for example, may be capable 

 of metabolizing a wide variety of organic 

 substrates including, let us say, sucrose and 

 lactose. If such bacteria are cultured for some 

 time in a medium containing sucrose but not 

 lactose, the presence of sucrase but not lac- 

 tase 1 can be demonstrated in the cells. When 

 lactose is added to the medium, therefore, 

 this sugar cannot be utilized at first. Gradu- 

 ally, however, in the course of about half an 

 hour, lactase begins to be produced by the 

 cells and lactose begins to be metabolized 

 more and more vigorously. Apparently the 

 presence of a particular substrate in the cell 

 has induced the production of the proper 

 enzyme. Presumably the particular gene ca- 

 pable of guiding the synthesis of the enzyme 

 had been present in the cells throughout but 

 this gene had remained dormant until stimu- 

 lated by the particular substrate. 



Induction of specific enzymes by specific 

 substrates, although studied most intensively 

 among bacteria, appears to be a general phe- 

 nomenon of widespread occurrence in many, 

 if not all, cells. Probably, therefore, such in- 

 duction plays an important role in mediat- 

 ing genie influences upon developmental 

 processes. 



EXPERIMENTAL EMBRYOLOGY 



A principal goal in the field of experi- 

 mental embryology is to reach a fuller under- 

 standing of what is sometimes called the 

 mechanics of development. In essence, the 

 experimental embryologist is trying to find 

 out how, when, and where the genes produce 

 their developmental effects. 



One perennial question continues to be: 

 How does differentiation occur? Usually all 

 the cells of a young embryo appear to be 

 alike. Yet marked differences soon appear as 

 the specialized tissues — nerve, muscle, carti- 

 lage, bone, and so forth — come into being, 

 and as the tissues become fitted together in 

 the final structure of the adult organs. 



1 Also called (3-galactosidase. 



Since all the cells of the embryo arise by 

 repeated mitosis from the original fertilized 

 egg cell, it seems probable that all must re- 

 ceive identical sets of genes. The possibility 

 exists, however, that somehow genie differ- 

 ences may arise among the embryonic cells. 

 Consequently, the experiments of Robert 

 Briggs and Thomas King of the Institute for 

 Cancer Research at the Lankenau Hospital 

 in Philadelphia are of particular interest. 

 These workers have succeeded in removing 

 the zygote nucleus from frogs' eggs, and in 

 substituting other nuclei, taken from the 

 individual cells of frog embryos at different 

 stages of development. Generally speaking, 

 this work indicates that up until the early 

 gastrula stage, at least, a nucleus from any 

 one of the many cells present in the young 

 embryo can be substituted for the zygote 

 nucleus without impairing the capacity of the 

 egg to fulfill its developmental destiny. Nu- 

 clei from later stages, however, are not suc- 

 cessful substitutes. But whether this failure 

 results from the reduced size of such nuclei, 

 or from some change in genie constitution, 

 remains an open question. At present there 

 does not seem to be any convincing evidence 

 to show that the various embryonic cells do 

 acquire distinctive differences in their genie 

 constitutions. It seems necessary, therefore, 

 to look elsewhere in trying to solve the prob- 

 lems of cellular differentiation. 



In general, it would seem, the genes re- 

 ceived by a particular embryonic cell deter- 

 mine the developmental potentiality of that 

 cell, but the environment of the genes and of 

 the cell as a whole determines the actuality 

 of development. Sooner or later the genes in 

 the different cells come under the influence 

 of many small differentials of local environ- 

 ment. Differences in relation to the position 

 of the cells in the embryo and differences in 

 the cytoplasmic composition begin to appear, 

 and these small differences seem to play a 

 crucial role in cellular differentiation. 



The cytoplasm represents a part of the en- 

 vironment of any set of genes; and a given 

 set of genes may produce totally different 



