The authors provide new estimates of the return to job seniority using a dataset similar to that employed in Joseph Altonji and Robert Shakotko, "Do Wages Rise with Job Seniority?" Review of Economic Studies (1987) and Robert Topel, "Specific Capital, Mobility, and Wages," Journal of Political Economy (1991). They consider the strengths and weaknesses of these studies' treatment of economy-wide trends, their dating conventions for tenure and wages, their handling of wage observations that might span multiple jobs, and their estimation approaches. Re-estimation points to an effect of ten years of tenure on the log wage equal to .11, which is above the preferred estimate of Altonji and Shakotko but far below that of Topel. Changes in earnings distributions and in the employment relationship motivate the examination of more recent data. Perhaps surprisingly, the return to tenure has probably increased over time.hether seniority has a large effect on W wage growth has been the subject of continuing controversy. At stake is the empirical relevance of theories emphasizing a role for worker-financed firm-specific capital in wage growth and turnover behavior, as well as models of wages that emphasize the use of deferred compensation as an incentive, insurance, or sorting mechanism (see Carmichael 1989; Hutchens 1989; Malcomson 1999; Gibbons and Waldman 1999; and Farber 1999). The size of the return to seniority is also important in assessing the costs of dislocation from work, a and participants in the Ashenfelter Festschrift conference at Princeton University for helpful comments. Without implying any endorsement of our analysis and conclusions, we give special thanks to Robert Topel for comments on a previous draft of the paper and for graciously providing us with a set of computer programs that greatly aided our research and to Paul Devereux for assisting us with the "full PSID sample" analyzed in the paper. We also thank Tecla Loup, Kate McGonagle, and Robert Shoeni for assistance with the PSID data. Finally, Altonji thanks Orley Ashenfelter for providing support and inspiration over many, many years.

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Much of the cell surface on the ciliate Tetrahymena thermophila is covered by a polarized lattice of cytoskeletal structures that are associated with basal bodies of the ciliary rows. Unique structural landmarks, including an oral apparatus and contractile vacuole pores, develop before cell division in localized domains located, respectively, posterior and anterior to the transverse fission zone. All of these structures can be visualized by specific monoclonal antibodies. A single-locus recessive mutation, disorganized-A (disA), primarily affects the striated rootlets of the ciliary-row basal bodies and brings about a severe disorganization in the positioning and orientation of these basal bodies and associated cytoskeletal elements. Nonetheless, the new oral apparatus, contractile vacuole pores, and other unique structures appeared at or near their normal sites along the anteroposterior axis of disA cells, indicating that the positioning of these localized structures is not dependent on the integrity of the ciliary rows. Abnormalities were present in the details of construction of some of the localized structures and in aspects of cell shape that may be influenced by these details. In the main, however, analysis of disA mutant cells indicates that intracellular domains near the cell poles develop independently of the vectorial polarity of the ciliary rows.

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