Variations in the rates of DNA and RNA synthesis during the DNA synthetic ( S ) period of the cell cycle have been determined in explants of early Rana pipiens embryos.Cells of dorsal axial ectoderm-mesoderm regions and belly endoderm regions of the embryo were partially synchronized using 5-fluorodeoxyuridine. The partially synchronized explants were incubated continuously or at intervals during the S period with radioactive nucleic acid precursors. Autoradiographic and biochemical evidence indicates that at the gastrula, neurula and tailbud stages the DNA is replicated discontinuously in two maxima with a slowing in the synthetic rate in the middle of the S period. There is an increase in the proportion of the DNA which replicates late as development proceeds from gastrulation. At the neurula and tailbud stages the proportion of late replicating DNA is greater in the belly endoderm than in the dorsal axial ectoderm-mesoderm.Experiments utilizing H3-5-uridine indicate that RNA is synthesized discontinuously in two maxima during the S period in both dorsal axial and belly regions at the earlier neurula stage. By the tailbud stage, however, a significant decrease in the second maximum of RNA transcription occurs. RNA extraction experiments indicate that these changes can be attributed, in part, to changes in the synthesis of DNA-like RNA. These findings are discussed in relation to cell determination.Gastrulation in developing amphibian embryos marks the time when the germ layers are formed and the fates of cells begin to be fixed. Cells of the early gastrula are undetermined and are competent to differentiate into various cell types. During gastrulation dorsal ectoderm and mesoderm cells are determined to form neural tissue, and somites and notochord, respectively (Holtfreter and Hamburger, '55). The endoderm cells are determined to form gut derivatives later during neurulation (Balinsky, '61 ). These processes of determination are paralleled by a gradual restriction in the competence of cells for various kinds of differentiation.A marked change in the synthetic activity of cell nuclei also occurs during gastrulation in that the length of the S period like RNA (D-RNA) and soluble RNA procedes that of ribosomal RNA which begins to be transcribed later during gastrulation and neurulation (Brown and Littna, '64). As cells of the germ layers become determinted, however, the rate of D-RNA synthesis decreases (Woodland and Gurdon, '68), and the number of different kinds of D-RNA molecules transcribed from redundant DNA sequences is reduced considerably (Flickinger, '70 a,b). These experiments indicate that genetic activity decreases as development proceeds from the neurula to the larval stage. Over the same period of time, more kinds of D-RNA molecules appear to accumulate in the cytoplasm (Denis, '66; Greene and Flickinger, '70).This investigation is aimed at examining the cause of the progressive restriction in

This paper offers an overview of the intellectual and social structure of science in the United States since the mid- 1960s. It argues that new directions for research are increasingly established by factors previously considered `external' to the inquiry process itself. These factors include changes in the ethos of academic science, social reorganization, governmental `megaprojects' and secrecy, and the enlarged dimension encompassed by Price's term `instrumentalism'. Taken together, these factors have restructured the indices of performance for doing science. Inquiry has become increasingly bound to a highly utilitarian conception of knowledge, to institutional goals and politicized reward systems, and to the technological infrastructure. Stylistic changes ensue, relating to how science is done, and to the relationship between theorization and experimentation. Given the altered circumstances for research, productive scientific inquiry at this historical juncture seems to be best fostered in diverse environments where corporate and academic scientists cooperate.

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Transglutaminase and ornithine decarboxylase activities have been assayed at intervals after partial hepatectomy in regenerating liver cells fractionated to obtain nuclear, cytoplasmic-particulate, and cytoplasmic-soluble fractions. Ornithine decarboxylase activity, localized entirely in the cytoplasmic fractions, undergoes a dramatic induction during the first 4 h after partial hepatectomy and remains elevated. This induction is very sensitive to inhibition by cycloheximide and actinomycin D, as previously reported. Transglutaminase activity is localized in both the cytoplasm and the nucleus with the highest specific activity in the nucleus. Nuclear transglutaminase activity approximately doubles in the first 2 h of liver regeneration, apparently as a result of a translocation of enzyme from the cytoplasm to the nucleus. Inhibitor studies indicate that the translocation is not dependent upon protein or RNA synthesis. In the first 2 h, actinomycin D slightly activates transglutaminase activity in the cytoplasmic-particulate and nuclear fractions. Only at 4 h after the onset of regeneration do actinomycin D and cycloheximide show some inhibition of transglutaminase activity indicating de novo synthesis at this time. A broad increase of transglutaminase activity occurs from hours 12-16 to hour 32 after partial hepatectomy in the nuclear and cytoplasmic-particulate fraction. These data suggest the existence of a function for transglutaminase in the nucleus of rat liver cells.

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The incorporation of tritiated nucleosides into DNA and RNA has been examined in partially synchronized cells of Rana pipiens embryos at the neurula and tailbud stages. Tritiated thymidine and deoxyguanosine are incorporated into the DNA in two maxima, or waves, during the S phase at both stages. More DNA replicates in the early maximum at the neurula stage than at the tailbud stage. A comparison of the degree of incorporation of labelled deoxyguanosine to labelled thymidine into DNA suggests that earlier replicating DNA at both stages may be GC-rich compared to later replicating DNA. The incorporation of tritiated uridine into RNA during the S phase also differs between the neurula and tailbud stages. Pulse and continuous label experiments indicate that at the neurula stage the highest rate of RNA synthesis occurs late in the S phase whereas at the tailbud stage the higher rate of RNA synthesis has shifted to an interval earlier in the S phase.The mechanism by which the emergence of new synthetic pathways occurs in differentiating cells seems to be related to DNA replication (9). It has been proposed that changes in DNA replication, or in a replication-related process, could provide a basis for the programming of differential gene expression in eukaryotic cells (1 1, 19). The phenomenon of obligatory DNA synthesis for gene expression and cytodifferentiation has been described in a number of eukaryotic cell systems (9,11,15,17). The dependance of gene expression on DNA replication is a common feature of the development of the larger bacterial and animal viruses (5,23). With the problem of the significance of DNA replication for differential transcription in mind, we have examined changes in the timing of DNA and RNA synthesis during the DNA synthetic (S) phase of embryonic neurula and tailbud cells of Rana pipiens. The data show a coincidence of changes in the synthesis of DNA and RNA during the S phase in embryonic cells, and they are consistent with the hypothesis that changes in DNA replicationa re related to changes in the timing of transcription in the embryonic cell. MATERIALS AND METHODSThe procedures have been described in detail previously (18). Neurula and tailbud embryos of Ranapipiens were used in this study. Stage 13 neurulae and stage 18 tailbuds were cut into dorsal axial ectoderm-mesoderm and belly endoderm regions in order to facilitate the entry of isotopes and to separate cell types with different prospective pathways of development (Fig. I). Groups of 50 cut explants were incubated in NIU-TWITTY solution (16) containing penicillin G(100 units) and streptomycin sulfate (50 pg/ml).M 5-fluorodeoxyuridine (FUdR) to inhibit DNA synthesis (8) after which lo-' M unlabelled thymidine was added to release the block of DNA synthesis. By this method cells accumulated at the beginning of the S phase traversed the rest of the cell cycle in synchrony.To induce cell synchrony the explants were incubated at 20°C for 14 hr in 11

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