| Abstract: | The ultrastructure of the Drosophila melanogaster embryo is described during the transition from the syncytial to the cellular blastoderm. The centriole is unusually short, only 150–175 mμ in length, and has a central cylinder and radial “spokes.” Although two centrioles have not been found together, a structure resembling a procentriole is frequently found with the centriole. Differentiations of cisternae of ER with the characteristics of annulate lamellae are found in the blastoderm adjacent to the yolk. Some evidence suggests that these differentiations first form adjacent to yolk nuclei during the blastema and early blastoderm stages and that subsequently they are incorporated into the blastoderm cells.A new supranuclear cytoplasmic region is formed in blastoderm cells after the nuclei have moved from the surface. Within this area a cytoplasmic complex forms consisting of Golgi, agranular tubules, and granular tubules and cisternae. The possible origins of the new ER are discussed. Definite differences in fine structure, both in the number of mitochondria and in the type of differentiation of the supranuclear complex, were found between the mid-dorsal and mid-ventral regions of the embryo. The differentiation of the supranuclear complex is discussed in relation to the establishment of the determinative nature of the embryo and to cellular differentiation. |
