CLEAVAGE IN-VIVO AND IN-VITRO IN THE MARSUPIAL MACROPUS-EUGENII

In the tammar wallaby, transport down the oviduct takes less than 24 h after fertilization and a mucoid coat is deposited within a few hours of fertilization, with excess spermatozoa trapped in the mucoid layer . The mucin coat thickens as the zygote passes down the oviduct. A pro teinaceous shell is laid down outside the mucin coat in the utero-tuba l region of the tract. The fertilized zygote enters the uterus in the pronuclear stage with cleavage proceeding in the uterus. In vivo, the first cleavage takes place two days post coitum (p.c.) (approximately 24 h after ovulation) but the next three cleavage stages may be comple ted within 24 h (between 48 h and 72 h p.c.). Thus, cell-doubling time appears to be around 8 h for 2-8-cell stages. Cleavage in vitro can o ccur with, or without, the shell membrane. Cleavage in early embryos o f the tammar in vitro is slower than that occurring in vivo, and in vi tro there may be a '4-cell block' in early development, as in dasyurid s. The pattern of cleavage differs markedly from that of dasyurid mars upials in that there is no extrusion of yolk material from the cells a nd no separation of the blastomeres during the first cleavage stages t o the 8-cell stage. The blastomeres are characterized by numerous vesi cular structures and lipid droplets, but no yolk bodies. Polarity is n ot marked in early cleavage, but by the 8-cell stage polarity has deve loped with surface microvilli and numerous granular vesicles and mitoc hondria in the cortical regions at one pole of the cells, but sparse m icrovilli on the inner surfaces and at the other pole. There are compl ex intervillous interdigitations of microvilli between cells. However, clear identification of cells as pluriblast or trophoblast cells is n ot possible up to the 8-cell stage examined. These results demonstrate that this macropodid marsupial has a distinctive pattern of early dev elopment which differs from that of Didelphis and of the dasyurid mars upials so far described.