I welcome the Editor's invitation to contribute to this anniversary symposium. My thoughts on the subject have wandered not in the direction of new theoretical concepts, but rather towards a set of reflections on the usefulness of the dual economy model. Like most other models this one has illuminated both more and less than was intended; more in the extent to which it. is used by some writeis to explain recent developments in mature industrial countries (notably Kindlebergcr, 1967;Cornwall, 1977), and less in the speed with which real wages have risen in some labour-rich developing societies, in defiance not only of the model but also of previous historical experience. These reflections are therefore mainly factual.The dual economy model comes in a wide range of specifications, since each writer is free to make his own assumptions. The version that I am using here has three characteristics. First, there are two sectors, hereinafter called "modem" and "traditional", such that the modem sector grows by recruiting labour from the traditional. Secondly, unskilled labour is paid more in the modem sector than in the traditional sector for the same quantity and quality of work. And thirdly, unskilled labour is initially abundant' in the sense that at the current wage much more labour is offered to the modem sector than that sector wishes to hire.The inodel focusses on explaining the distribution of income inside the modem sector, and therefore the rate of growth of the modern sector, but it has many other ramifications.This essay will not concern itself with the mechanics of the model; or with comparisons between this and the numerous other versions of two-sector models. It will concentrate on three general topics :The effects of the modem and traditional sectors on each other. The sources of labour abundance. The maintenance of differentials in the labour market.
2. 3.1The phrase "surplus labour" is not used in this essay since it causes emotional distress. As always, the idea intended to be conveyed is that of an in6nitely elastic supply of labour to the modern sector at the current wage. 21 1
WITH 8 PLATES AND 27 TEXT FIWBES.The purpose of the following paper is a description of various typical stages in the development of the back, the limbs, and the body-wall in man. The work is based primarily upon reconstructions, according to the method of Born,' of parts of five human embryos; it has been extended and controlled by a study of the external form and of serial sections of several other human embryos. Dr. Lewis has devoted special study to the formation of the arm, Dr. Bardeen to that of the leg, the bodp-wall and the back. I n the accompanying table a list is given of the embryos utilized.Those marked with an asterisk have been reconstructed.We shall consider the early stages in the development of the limbs, the body-wall and the back, first, from the point of view of the external form and, secondly, from that of internal structural differentiation.
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