EmbryoIntroFIG 4: Embryos begin development inside the mother.
Top panel. Sagittal view of an adult hermaphrodite at the midbody where the uterus contains a row of developing embryos along the body length (3 can be seen here). The intestine runs more dorsally, just above the uterus. The vulva can be seen as a narrow gap along the ventral hypodermis, with a ridge of thickened cuticle closing the vulval opening. Depending on the rate of egg-laying, embryos generally develop inside the uterus for about 150 min before they are expelled via the vulva. Afterwards each embryo continues development in the outside environment. This animal was prepared by high pressure freeze fixation and freeze substitution. (Image source: N843 [Hall lab] 0444.)
Bottom panel. Closeup of one of the same embryos, showing six cells floating inside the blastocoel, surrounded by the eggshell. This embryo probably has about 12-18 cells in total, all of which are still early blastomeres. Their cytoplasm is dominated by large nuclei, many small dark yolk granules, mitochondria and circular vacuoles. Two central cells appear to be connected by a wide cytoplasmic bridge, suggesting that they are in late stages of mitosis. (Image source: N843 [Hall lab] 0445.) Note that each embryo in this figure is floating in a sea of fluid (the blastocoel), while prior to gastrulation, there is no fluid space separating neighboring cells inside the cell mass. Later, as gastrulation proceeds, much of this fluid will be pulled inside the embryo to initiate formation of the pseudocoelom.
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