. the fertilisatiorj of egg by sperm which occurs in the ordinary sexual reproduction of man and most animals. First there is the fusion of two separate nuclei, two single sfts of chromosomes, and the consequent shuffling and recombination of the hereditary factors. In the second place, there is what is usually called aclivatioi—the starting-off of the egg upon its career of grow:h and development. In most species, if fertilisatbn does not take place, the egg remains inert, and sooaer or later perishes. However, in some animals, such as Aphids (plant-lice) and water-fleas, the egg is capable o
. the fertilisatiorj of egg by sperm which occurs in the ordinary sexual reproduction of man and most animals. First there is the fusion of two separate nuclei, two single sfts of chromosomes, and the consequent shuffling and recombination of the hereditary factors. In the second place, there is what is usually called aclivatioi—the starting-off of the egg upon its career of grow:h and development. In most species, if fertilisatbn does not take place, the egg remains inert, and sooaer or later perishes. However, in some animals, such as Aphids (plant-lice) and water-fleas, the egg is capable of developing without this stimulus. Such foms are called parthenogenetic. The bee is of especial interest, since the males, or drones, are father- less, produced parthenogeneticaUv, while the queens and wo'kers all arise from fertilised eggs. In the course of evolution, a change must have come about so that activation is no longer carried out by the sperm, but by some other means. What in these ^^i> Chrtmosomes («) of norual female Dro- sophila. Reproductd from " Mendelism,' Fig. 3. {b) Of normal male. (c) Of au XXY female individual. by Prof. R. G. Punnell, , by permission Fig. 2.—sex-linked inheritakce of white eye in DROSOPHILA. Females on left, males on right. The sex-chromosome behaviour is represented diagrammatically ; the chromosome bearing the factor for red eye is represented in black, that bearing the factor for white eye in outline. Reproduced from " ; by Prof. R. G. Punmlt, , by permission of the Author and of Macmillan & Co., Ltd. and even good when the stock's hereditary constitu- tion is a good one. The ecclesiastical prohibition of the marriage of near relatives is thus seen to be only conditionally justified on biological grounds.' • The stimulating effect which often follows a cross between markedly different stocks, while also explicable in terms of factors, is due to rather complex causes. The reader is re-
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