Fig. 2. Oogenesis and early embryogenesis in C. elegans. (A) A schematic of an adult C. elegans hermaphrodite to show the bilaterally symmetrical U-shaped gonad arms (grey). Anterior is towards the left, dorsal is upwards. The major anatomical features are indicated. (B) One arm of the U-shaped C. elegans gonad. After dividing mitotically in the distal arm of the gonad, germ cell nuclei (in blue), partially enclosed by membrane cubicles, migrate proximally, enter meiosis and cellularize into oocytes. Mature oocytes enter the spermatheca, are fertilized and exit as zygotes into the uterus. The fertilized zygotes form an eggshell, finish meiosis and undergo the first mitotic divisions in the uterus. (C) (a) Meiotic divisions I and II, and (b) the first two mitotic divisions of a C. elegans embryo. DNA, blue; meiotic and mitotic spindles, green. (a) After fertilization, the C. elegans embryo executes the first and second meiotic division in the anterior of the zygote, resulting in the extrusion of two polar bodies and the formation of a haploid female pronucleus (top). The female meiotic spindles are small and lack centrosomes. After meiosis, the male and female pronuclei fuse, and the zygote enters mitosis. (b) The mitotic spindle is organized by two centrosomes (green circles) at the spindle poles. The first division is asymmetric, producing larger anterior and smaller posterior daughter cells that differ in cell cycle timing and mitotic spindle orientation during the next round of mitosis.