Meiosis and Fertilization in Ascaris

This page features a series of photographs of Ascaris sp. (a type of nematode) showing the stages of fertilization and meiosis. The reason these have been grouped together here is because meiosis does not occur in Ascaris without fertilization (fertilization before meiosis is unusual in the animal world).

		Sperm Entrance The non-flagellated haploid sperm (arrows) have entered the eggs. Meiosis can only occur after this has happened. Note the lack of layers which will be seen to surround the egg later.
		First Meiotic Division The first meiotic division has started (dark area in the lower part of the egg), while the sperm pronucleus can still be seen (dark area in the upper middle part of the egg). Note the fertilization membrane and associated layers.
		Second Meiotic Division The second meiotic division (dark area in the lower right area of the egg) is now seen to be occurring in the same location as the first meiotic division (seen in the previous slide). The sperm pronucleus can still be seen as a dark area in the centre of the egg. Note the first polar body (arrow), which was released as part of the completion of the first meiotic division and is located at the inner edge of the chitinous layer.
		Completion of Meiosis The two haploid pronuclei (one male and one female) are seen in the centre of the cell. The nuclear membranes of both pronuclei will now break down releasing their chromosomes. This forms the diploid zygote which proceeds immediately into the first cleavage division. Note the presence of two polar bodies, the second having been released following the completion of the second meiotic division. The dark structure along the bottom of the picture is the wall of the uterus. The fertilization shell layers are labelled as follows: 1. The fertilization membrane with outer protein layer 2. The chitinous layer 3. The ascaroside layer and perivitelline space

Some Other Sites Explaining Meiosis

Send comments to:
Sandra K. Ackerley (ackerley@uoguelph.ca),
Department of Intergrative Biology, University of Guelph, Guelph, Ontario, Canada N1G 2W1.