At the beginning of its development, growth of the embryo is slow, and in very early stages it is merely a rounded mass of cells. Later, the meristems of the epicotyl (stem or top) and root axis develop, but the whole embryo is still microscopic in size. Still later the cotyledons (seed leaves) start development from the apical meristem and their growth in length is rapid, but they are very thin and follow the contours of the seed coat. Growth in length of the cotyledons may be arrested by unfavorable nutritional conditions during the time of elongation. In such case, the lobes of the cotyledons may not attain the full length of the seed coat, or pellicle, which surrounds them. After the cotyledons have attained full length, growth in thickness begins in the area nearest the epicotyl and proceeds toward the margins. This growth in thickness results from cambium-like meristem with the formation of new cells. The formation of well developed or solid kernels that completely fill the cavity within the shell is dependent upon meristematic activity continuing almost to maturity. The weather conditions, the nutrition of the tree, or other factors that affect the synthesis and translocation of elaborated food materials from the leaves and shoots to the kernels at this time determine the degree to which the cotyledons are thickened, or in other words how well the nuts are filled. +Periods of Development+ In the development of the nuts there are three periods or stages: (1) The period of growth in size; (2) the period of nut filling or development of the kernel; (3) the period of maturing. What takes place during these periods of development determines the size the nuts attain, the degree to which they are filled, and finally the quality at harvest. These three developmental stages are interdependent, because the size of the nuts may affect the degree of filling, and that, in turn, the time and nature of their maturity. They are not entirely separate and distinct but overlap in that there is more or less development of the kernel, varying with the species, while the nuts are growing in size. In general, however, there is not appreciable kernel development until after the nuts have attained approximately full size, except in the chestnut. The outstanding example of this situation is the pecan. There is practically no growth of the kernel until after the shell of the nut has started to become hard. At that time growth of the embryo, which constitutes the kernel, become rapid. The major portion of the kernel is formed during a period of approximately one month, starting at Beltsville, Maryland about the middle of September. The final stages of filling occur just before the nuts mature, and the first nuts to fall usually have the best filled kernels. Later maturing nuts are generally poorly filled; their shells and kernels are often discolored, and the shucks fail to open properly, if at all.