"The politics of immigration in liberal democracies exhibits strong similarities that are, contrary to the scholarly consensus, broadly expansionist and inclusive. Nevertheless, three groups of states display distinct modes of immigration politics. Divergent immigration histories mold popular attitudes toward migration and ethnic heterogeneity and affect the institutionalization of migration policy and politics....I begin by discussing those characteristics of immigration politics found in all liberal democracies. I then investigate the distinctive modes of immigration politics in the three subsets of Western democratic states with distinctive immigration histories. I conclude by considering whether these three patterns will persist or how they might change as a result of future migration pressures and the further institutionalization of immigration politics and policies in Europe." Comments by Rogers Brubaker (pp. 903-8) and a rejoinder by the author (pp. 909-13) are included.

Developmental problems relating to the stability of cell phenotype, and to the interactions between cells during morphogenesis can be approached particularly well through the study of regenerating systems. In the case of lens regeneration the anatomy of the eye is such that it is possible to identify the tissue types that can form lens, and to analyze the role of the factors which control the regenerative process. At present, it has been established that the lens can regenerate from the iris (Stone, '59; Reyer, '54, '62); and there is some evidence that the lens can regenerate from the cornea (Reyer, '54).The most convincing evidence for lens regeneration from the cornea has been provided by Ikeda ('36, '39) for late embryonic and early larval stages of Hynobius unnangso. In this series of experiments, the lens anlage with the surrounding ectoderm was removed from embryonic animals and head ectoderm was allowed to heal over the eye cup. A histological study of these animals showed various stages of lens formation from the ectoderm over the eye cup, but a complete series of stages showing the process of lens regeneration was not presented. The time during which lens regeneration could take place was limited to a short period of embryonic life after lens induction. These results were complicated by the claim that lens regeneration from the iris takes place during the same time period (Ikeda, '37). In the second experiment in this series pieces of prospective and differentiating cornea from animals at different developmental stages were implanted into the eye cup of young animals after lens removal. Here lens formation was observed from the inner cell layer of the epithelial component of the cornea. The portion of the corneal epithelium which did not contribute to the lens usually developed into epidermis. This paper describes lens regeneration from the cornea in another species, XenoUniversity of Chicago, Illinois p u s laevis. The study was made on three different larval stages, and young metamorphosed animals. With this material the process of lens regeneration was studied histologically, the growth of the lens was plotted, and the competence of the eye to regenerate a lens at different developmental stages was measured. A cytological analysis was then made of the changes in the behavior of the nucleolus that take place during regeneration.This description of lens regeneration was supplemented by experiments designed to verify the observation that the cornea forms lens. This was done with extirpation and grafting experiments, in some of which tritiated thymidine was used as a marker.Two factors which control the process of lens regeneration were also studied. The role of the optic cup was studied by testing for lens regeneration in its absence. The limitation of regenerative capacity to the larval eye was studied by local application of thyroxin.A preliminary account of this work has been published (Overton and Freeman, '60; Freeman and Overton, '61, '62). MATERIALS AND METHODS AnimalsThe animals used for the e...

In annelids, molluscs, echiurans and sipunculids the establishment of the dorsalventral axis of the embryo is associated with D quadrant specification during embryogenesis.This specification occurs in two ways in these phyla. One mechanism specifies the D quadrant via the shunting of a set of cytoplasmic determinants located at the vegetal pole of the egg to one blastomere of the four cell stage embryo. In this case, at the first two cleavages of embryogenesis there is an unequal distribution of cytoplasm, generating one macromere which is larger than the others at the four cell stage. The D quadrant can also be specified by a contact mediated inductive interaction between one of the macromeres at the vegetal pole with micromeres at the animal pole of the embryo. This mechanism operates at a later stage of development than the cytoplasmic localization mechanism and is associated with a pattern of cleavage in which the first two cleavages are equal.An analysis of the phylogenetic relationships within these phyla indicates that the taxa which determine the D quadrant at an early cleavage stage by cytoplasmic localization tend to be derived and lack a larval stage or have larvae with adult characters. Those taxa where the D quadrant is specified by induction include the ancestral groups although some derived groups also use this mechanism.The pulmonate mollusc Lymnaea uses an inductive mechanism for specifying the D quadrant.In these embryos each of the four vegetal macromeres has the potential of becoming the D macromere; however under normal circumstances one of the two vegetal crossfurrow macromeres almost invariably becomes the D quadrant. Experiments are described here in which the size of one of the blastomeres of the four cell stage Lymnaea embryo is increased; this macromere invariably becomes the D quadrant. These experiments suggest that developmental 205 206 Freeman and Lundelius change in relative blastomere size during the first two cleavages in spiralian embryos that normally cleave equally may have provided a route that has led to the establishment of the cytoplasmic localization mechanism of D quadrant formation.

The genetics of body asymmetry inLymnaea peregra follows a maternal mode of inheritance involving a single locus with dextrality being dominant to sinistrality. Maternal inheritance implies that all members of a brood have the same phenotype, however, some broods contain a few individuals of opposite coil. One purpose of this paper is to explain the origin of these anomalous individuals. Genetic analyses of sinistral broods with a few dextral individuals have led to the development of a cross-over model, with the anomalous dextrals originating as a consequence of crossing over either during meiosis or mitosis in the female germ line. The crossover either reconstitutes the dextral gene from previously dissociated parts, or creates a dextral gene by means of a position effect. The probability of a crossover event depends upon the appropriate combination of complementary sinistral chromosomes. Each crossover event has the potential of creating a unique dextral gene. Genetic analyses of dextral broods containing a few sinistral individuals have demonstrated that different dextral genes vary in penetrance.The dextral gene produces a product during oogenesis which influences the pattern of cleavage in the embryo; this cleavage pattern is translated into the appropriate body asymmetry. The other purpose of this paper is to provide an assay for this gene product. Cytoplasm from dextral eggs injected into uncleaved sinistral eggs causes these eggs to cleave in a dextral pattern. Cytoplasm from sinistral eggs has no effect on the cleavage pattern of dextral eggs. While the dextral gene product is made during oogenesis, it does not function in controlling cleavage until just before this process begins.

How much variation is there in immigrant incorporation policies and practices across the Western democracies? Concluding that the effort to capture variation in typologies of incorporation schemes is likely to prove both futile and misleading, I propose a radically dis‐aggregated perspective that conceives of incorporation as the product of the intersection of migrant aspirations and strategies with regulatory frameworks in four domains — state, market, welfare, and culture. Because some but not all of the regulatory institutions in these domains were created with immigrant incorporation in mind, national incorporation frameworks are not fully cohesive, are constantly changing, and at best can be described as belonging to a handful of loosely connected syndromes.

National welfare states are compelled by their logic to be closed systems that seek to insulate themselves from external pressures and that restrict rights and benefits to members. They nonetheless fail to be perfectly bounded in a global economy marked by competition, interdependence, and extreme inequality. This article explores the consequences of transnational flows of labor both for the status of migrants who move to welfare states and for the viability of welfare states themselves. The consequences of migration for the fiscal and political stability of welfare states are discussed, and it is argued that migration has contributed to the Americanization of European welfare politics. It is concluded that the relatively free movement of labor across national frontiers exposes the tension between closed welfare states and open economies and that, ultimately, national welfare states cannot coexist with the free movement of labor.

A vigorous tradition in comparative politics argues that national policymakers develop characteristic and durable methods for dealing with public issues, that these can be linked to policy outcomes, and that they can be systematically compared. More recently, a number of scholars have suggested reversing the direction of causality, claiming that the nature of political issues themselves causes the politics associated with them. This policy sector approach implies that there should be cross-national similarities in the way issues are treated, whatever the styles particular nations adopt. The two approaches need to be integrated into a common framework built around a research strategy that investigates policymaking within specific sectors across multiple national cases. Such an approach can transcend the often sterile debate over whether the policies of nations are unique or are converging by seeking to explain how the nature of issues structures the variation among the policies of nations.

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