The fine structure of astral poles during both meiotic and early mitotic divisions of Lymnaea stagnalis eggs is studied. It is shown that both meiosis I spindle poles contain single centrioles due, apparently, to uncoupling of the nuclear and centriolar cycles before meiosis: DNA replication proceeds, but reduplication of the centrioles is absent. The outer pole of the meiosis II spindle also possesses one centriole and the inner pole contains a sperm basal body. The inner pole centrosome of the meiosis II spindle reveals a complex structure. Its periphery is formed as a result of transformation of the inner pole centrosome of the meiosis I spindle, and the center includes the sperm basal body. Thus, the zygote centrosome is a hybrid consisting of both maternal and paternal material. Both poles of the first mitosis spindle contain single centrioles. At the proximal end of each centriole an electron dense globule is found. The sperm basal body has not been found. Therefore, formation of the centrioles de novo during the first interphase may be proposed. At the second mitosis a pair of centrioles is observed at each spindle pole. Two rounds of centriole duplication appear to occur during the second interphase. As a result, the correlation between DNA content and number of centrioles typical for proliferating animal cells is restored. Later, such a correlation is maintained: at the third mitosis each spindle pole contains a diplosome. (C) 1994 Academic Press, Inc.