One of the obvious ways of testing the clonal selection hypothesis is to determine the minimum number of cells needed to initiate a measurable immune response. At least in the uncompromising, and now classical, hypothesis of one antigen-sensitive cell clone per antibody specificity (1, 2) it is mandatory that a given antigenic determinant should not be recognizable by every small lymphocyte--accepting that antigen-sensitive ceils are to be found among this, in several ways, heterogenous cell population. While this extreme result, if found, would truly invalidate the hypothesis it is unfortunately not possible to define exactly the maximum frequency of antigen-sensitive cells which would still be compatible with the hypothesis. Clearly, the more complex the immunological universe is thought to be, the higher becomes the number of different clones required by the hypothesis, and the lower becomes the expected frequency of a cell belonging to one particular clone. Few immunologists would probably expect a frequency higher than 10 -4.For reasons which were suggested by earlier studies of the factor of immunization (3), the strong histocompatibility antigens were considered to provide a particularly exacting test of the clonal selection hypothesis. The graftvs.-host (GVH) reactions in chicken embryos injected intravenously with adult chicken lymphocytes were used in the present study to determine the frequency of antigen-sensitive cells with re3pect to the strong antigens of the B locus (4). It is concluded that their frequency is too high to be compatible with the orthodox version of clonal selection.

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Hydatid disease of bone is rare. Ivanissevich (1934) gave an incidence of 2 per cent of all forms of hydatid disease in South America, but in Australasia it is less than I per cent. Ninety new patients with hydatid disease are admitted to New Zealand hospitals every year and the death rate from it in New Zealand is &23 per million. This is twenty times as great as that in the United Kingdom, and is attributable to the remarkable disproportion between the human population (two million) and the sheep population (forty-seven million). Forty-three cases of hydatid disease of bone are recorded in the Louis Barnett Hydatid Registry

967 968 PRIMARY IMMUNE RESPONSE IN GRAFTED CELLS Materials and MethodsAnimals.--Outbred eggs (preliminary experiments) were a cross between closed flocks of Rhode Island Red and Light Sussex (purchased from the Appleby Farm Ltd., Ashford, Kent, England). Highly inbred lines of White Leghorns, I and W, were maintained by Dr. D. G. Gilmour at the School of Agriculture, University of Cambridge, from where we obtained birds for production of F1 hybrid embryos. Fertility was low, hence we were much restricted in supply of this material. Through the courtesy of Thornbers Ltd., Halifax, Yorkshire, England, ample supplies became later available of birds and eggs which were of known genotype with respect to the powerful histocompatibility and blood group locus B (5-8).Chromosome Preparations.--Colcemid-treatedsplenic ceilsweresuspendedin 0.95% sodium citrate and kept at 37°C for 20 rain. After slow spinning they were transferred to ethyl-acetic fixative (ethanol 3, glacial acetic acid 1) for ~ hr and spun again and transferred to methylacetic fixative (methanol 3, glacial acetic acid 1). The cells were then cold spread on to clean microscope slides and stained with 2% propionic orcein. Permanent preparations were made. This technique has given uniformly good preparations and the male cells of ZZ chromosome constitution were readily distinguished from female ceils which have only one large Z mediocentric chromosome.