The development of the first meiotic prophase stages was studied in two series of human female embryos and fetuses aborted for social reasons. The first series (64 embryos or fetuses aborted at 6-24 weeks of gestation) was used mainly to perfect the methods applied to obtain chromosome preparations and synaptonemal complex spreads. The second series (37 embryos or fetuses aborted at 9-24 weeks of gestation) was used to establish the timing and to characterize the different stages of prophase I. Leptotene-zygotene figures were observed in some embryos at 10 weeks of gestation. Typical zygotene figures were seen at 11-22 weeks. Pachytenes were first observed at 12-13 weeks, and the proportion of these figures was usually lower than 40%. Diplotenes were seen in fetuses with a gestational age of 14 weeks or more. The duration of the process in the human female is thus about 3-4 weeks, a similar period to that described for the male.

We describe the behaviour of synaptonemal complexes (SCs) in Syrian hamster and mouse oocytes. In Mesocricetus auratus, synaptonemal complexes can be observed from birth up to 7 days of life. In foetuses from Mus nTusculus, synaptonemal complexes can be observed from the 14th day of gestation up to the first day post-partum, when the cells enter the dictyotene stage. In both species, synaptonemal complexes show, in general, the same morphology described in male cells by light and electron microscopy, with the exception that the axes of the sex bivalent are not identifiable. The leptotene stage can be identified although it is probably of short duration. Only one type of zygotene (zygotene II of Dietrich and Mulder (Chromosoma 88: 377), 1983) has been observed. In the hamster we also describe a desynaptic diplotene stage previous to the desintegration of the SCs. In oocytes from both species late pairing (or precocious separation) of a single bivalent can be seen in a few cells. Interlocking of some bivalents with delayed pairing of the affected region is rather frequent. Furthermore, hamster oocytes may show heterosynapsis of the telomeres of autosomal bivalents by pachytene.

The progression of first meiotic prophase and synaptonemal complex (SC) formation in female rats, Rattus norvegicus S.D., is described through the analysis of the different stages of the first meiotic prophase, and confirms the high synchrony of the process in this species. Leptotene is a stage of very short duration and since pairing of the homologues begins very early, only a leptotene-zygotene stage can be distinguished. The progression of pairing during zygotene is asynchronous. The morphology of the SCs is similar to that described in other species. During diplotene and before disintegration of the lateral elements, desynapsis takes place. In some oocytes a double or even multiple nature of lateral elements was seen. Associations between SCs and nucleoli or nucleolar filaments are frequent. The presence of fragmented SCs can be interpreted as a technical artifact.

Meiotic studies in 15 female puppies (Canis familiaris) revealed that in this species first meiotic prophase begins postnatally. Leptotene is observed until day 5, zygotene from day 5 to day 15, pachytene from day 10 to day 30, diplotene from day 15 to day 30, and dictyotene appears by day 30. These results are not in agreement with previous reports on the timing of oogenesis in this species. Key words: Canis familiaris, oogenesis.

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