Early ontogeny of the annual fish genus Nothobranchius: Cleavage plane orientation and epiboly

Living embryos of ten species of the teleost fish Nothobranchius were observed from fertilization through the ten-somite stage. Cellular behaviour during early development showed differences as well as similarities among species. In one species at the eight-cell stage the blastomeres are arranged in two rows of four cells, all in one plane. In three other species the blastomeres are arranged in two layers of four cells, each one above the other, and in the remaining six species the arrangement is intermediate between the one-layer and two-layer pattern. Hybridization experiments showed that the sequence of orientation of cleavage planes is not under the control of either the nucleus or of cytoplasmic factors from the sperm. At midcleavage stages, marginal cells of the blastoderm undergo fusion, thus giving rise to the external-Yolk Syncytial Layer. A second period of fusion of cells of the Enveloping Layer occurs just before epiboly starts and a third one after epiboly has concluded. Marginal cells of the Enveloping Layer do not possess cell protrusions such as blebs, lobopodia, or filopodia; therefore, spreading of the Enveloping Layer during epiboly by means of locomotion of marginal cells seems unlikely. Evidence is presented that the deep cells within the segmentation cavity remain separated from one another through contact inhibition during epiboly and that they use the Enveloping Layer as a substratum during their dispersion over the yolk. A description of normal stages applicable to all ten species of Nothobranchius is presented.