الفهرس 
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Abstract
Five different varieties of squash cucurbita pepo L. were used in a complete diallel crosses mating system to generate experimental populations of F1 hybrids and F1 reciprocal hybrids. The F2 generations of all F1 hybrids and their reciprocals were also obtained. These varieties were the American varieties, Black Beauty, Caserta, Table Queen, Early Golden Bush Scallop and the Egyptian variety El-Askandarani. Therefore, the genetic materials included: 5 parents, 10 F1 hybrids, 10 F1 reciprocal hybrids (F1r)’ 10 F2 generations and 16 F2 reciprocal generations (F2r). All 45 genotypes were evaluated in a field trial experiment at the Experimental Station, Faculty of Agriculture, Mansoura University in the Summer and Nili seasons of 1979. The objectives of this investigation were primarely initiated to determine the amount of heterosis, reciprocal effect, inbreeding depression, general combining ability (GCA), specific comb!ning ability (SCA) and heritabilities. I- Heterosis : The estimates of heterosis from both seasons were similar, therefore it could be valuable to present the results obtained from the combined data. A) Vegetative traits at seedling stage : l) The manifestation_ of heterosis for F1 hybrids _over mid-parents was appearent for all traits. A rather high value of heterosis of 115.45% .was reported for fresh weight of seedling. The lowest value of heterosis was 6.09% for cotyledonry leaves shape index. 2) Heterosis values for F1 reciprocal hybrids (F1r) over mid-parents were significant for cotyledonry leaves shape index (2.54%), hypocotyl diameter (24.00%), fresh and dry weight of seedling (110.91% and 81.82%). The largest heterosis values at seedling stage appeared mostly for fresh and dry weight. The estimated values of heterosis ranged from 0.39% to 110.91% for hypocotyl height and Fresh weight of seedling, respectively.