Embryos derived from spg^unspecified mutant females show abnormal cleavage.

Cellularisation of blastoderm embryos derived from homozygous mothers usually occurs only at the anterior and posterior poles, although isolated groups of cells may occasionally be found in the ventral midregion. Posterior blastoderm cells appear relatively normal, consistently forming columnar cells with elongated nuclei. Regions of cytoplasmic continuity between the posterior blastoderm cells and the yolk sac are broader than in wild-type embryos. The nuclei remain spherical in the midregion and an irregular layer of cytoplasmic material is present at the surface of the embryo. The posterior midgut rudiment (PMG) invaginates, although the cells appear abnormal. The cellular organisation of the PMG becomes increasingly abnormal, resulting in a stratified layer of irregularly shaped cells, some of which contain yolk particles. The PMG stops extending when it has reached a point approximately midway along the dorsal surface of the embryo, at which time the PMG and the yolk sac show extensive tissue disruption.

Homozygous embryos derived from heterozygous females are morphologically normal and viable. 90% of heterozygous and homozygous embryos derived from homozygous females appear to progress normally through the syncytial blastoderm stage, except for occasional asynchrony during the last three syncytial divisions and a reduction in the number of pole cells from the normal 35 cells to about 20 cells. Major abnormalities become evident during the transition from syncytial to cellular blastoderm; blastoderm cells are formed at the anterior and posterior ends of the egg, but the midregion, covering about 70% of the total blastoderm, remains noncellular. The last syncytial division is retarded in the midregion. The nuclei in this region fail to elongate and subsequently move away from the surface, aggregate and degenerate. The blastoderm layer thickens along the ventral midline, and in the posterior region the pole cells and blastoderm cells migrate together in an anterior direction along the dorsal surface, about 1/4 of the length of the egg and invaginate, forming a pocket of cells that resembles the pole cell invagination in a normal embryo. At about the same time, the cells in the anterior region migrate in a posterior-dorsal direction, and later an invagination occurs at the ventral surface of the anterior pole that resembles the stomodeal invagination in normal embryos. The cells at both ends of the embryo appear to proliferate and undergo some development in older embryos.