||In higher plants the development of the female gametophyte - the embryo sac - involves two subsequent processes: megasporogenesis and megagameto genesis. Megasporogenesis is the process during which one functional mega spore is formed by meiosis of one particular nucellus cell. This functional mega spore can contain one, two or four haploid nuclei (respectively following the mono-, bi- or tetrasporic developmental type) depending on the circumstance whether a cell-wall is formed after the first and/or the second meiotic division or not. Additional megaspores, when formed, degenerate during megasporo genesis. The functional megaspore develops into the embryosac after a number of mitotic divisions. This process is called megagametogenesis. The cell in volved is called megasporocyte or megaspore mother-cell during megasporo genesis and forms a megagametophyte during megagametogenesis.In this thesis the ultrastructural aspects of megasporogenesis have been studied with the employ of electron-microscopic technics. Initially we intended to study the megasporogenesis in three species, each representing one of the three types mentioned above. For this purpose the species Impatiens walleriana Hook. f, Allium cepa L. and the Lilium hybrid 'Enchantment', were chosen which, according to other investigators respectively would show the mono-, bi- and tetrasporic types of development. However, our observations revealed a bisporic type of development for Impatiens as well, so that eventually two species having the bisporic type and one species having the tetrasporic type were examined.During our investigation the ultrastructural changes and the localization of the various cell organelles in the different developmental stages were observed. At the same time quantitative data concerning the number of the various cell organelles were collected during the whole of the developmental process. See enclosures I, II, III, IV and V.In the chapters 3, 4 and 5 the ultrastructural aspects of megasporogenesis respectively of Lilium, Allium and Impatiens are described and discussed separately. From the results we can conclude that each species shows its specific ultrastructural aspects during megasporogenesis. Lilium shows a very specific formation of an extensive endoplasmic reticulum, whereas in Allium the dictyosomes show a typical ultrastructure and localization. Impatiens shows an increasing amount of starch granules in the plastids, which is not found in the other two species.The differences and similarities in the ultrastructural changes of the cell organelles found in the three species are discussed more in detail per cell organelle in chapter 6. In chapter 7 a classification is given of these similarities and differences in different groups. Some of the similarities in the ultrastructural aspects of megasporogenesis between the three species examined seem to be specific for the process of meiosis, because of a known or presumed functional relation to this process. The other similarities and all the differences are specific for the process of megasporogenesis. The differences can be distinguished in species- specific and (developmental) type-specific characteristics.The similarities in the ultrastructural aspects of the three species, which are related to the process of meiosis, are in the first place found in the development of the nuclear chromatin and of the microtubules. The chromatin structure in the nucleus of each species shows similar ultrastructural changes, owing to the different developmental stages. The microtubules have a function in the spindle figure. Secondly, the ultrastructural changes of the nuclear envelope, the nucleolus, the nucleolus-like bodies in the cytoplasm and the cytoplasmic ribosomes show similarities between the three species examined. The undulations of the nuclear envelope, probably caused by a synthesis of new nuclear membrane before nuclear division and the sacculation of the inner nuclear membrane are probably related to a nucleo-cytoplasmic exchange of information, which takes place from nucleus to cytoplasm or vice versa during meiosis. The decrease of the number of ribosomes per cytoplasmic area during meiotic prophase is related to a low number of nuclear pores and an increase in volume of the nucleolus at this stage; the increase of the number of ribosomes per cytoplasmic area after metaphase I is related to the disintegration of the nuclear envelope before metaphase I and the presence of nucleolus-like bodies in the cytoplasm after metaphase I.Some similarities related to the process of megasporogenesis are the position of the developing megasporocyte in the nucellus, the localization of the plasmodesmata in the cell-wall and the storage of reserve-food in the cell. The developmental changes of the mitochondria, the plastids and the appearance of the central vacuole are related to both megasporogenesis and megagametogenesis. During megasporogenesis the developing megaspore mother-cell is surrounded by nucellus cells and is dependent on these cells for its nutrientsupply. There seems to be an interaction between the nucellus cells and the developing megaspore mother-cell, as a result of which the development of the megaspore mother-cell is influenced by the nucellus. A direct contact between the megaspore mother-cell and the nucellus cells is found only on the chalazal side of the megaspore mother- cell, where plasmodesmata are found in all three species examined. On the chalazal side of the nucellar tissue the vascular bundle ends and from this side a nutrient transport to the megaspore mother-cell takes place, causing a nutrient-gradient in the cell. This nutrient-gradient may define the polar distribution of some cell organelles in the cell. In the megasporocyte a storage of reserve-food takes place. The degree of heterotrophy of the megasporocyte can be determined by this storage of reserve-food and seems to be species-specific. During megasporogenesis the mitochondria and the plastids show ultrastructural changes, whereas they return to their original ultrastructure during megagametogenesis. The formation of a central vacuole during megagametogenesis is also related to the presence of small vacuoles during megasporogenesis.Apart from these general characteristics of the process of megasporogenesis we can distinguish species-specific and type-specific characteristics of the process. The species-specific characteristics are specific differences in the contact with the chalazal neighbour nucellus cells and in the reaction of the nucellus cells on the presence of the megasporocyte and the developing megagametophyte. The mitochondria and the plastids show specific ultrastructures, whereas the storage of reserve-food in the formation of starch and lipid bodies seem to be speciesspecific. The type-specific characteristics are concerned with the classification of the process in the bi- and tetrasporic types of development. The ultrastructural changes of the dictyosomes and of the endoplasmic reticulum, the polar localization of some cell organelles and the degeneration of the non-functional cell are specific characteristics related to the type of megasporogenesis.