Developmental Biology ONLINE!

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 (this is unusual in the animal world). As with the mitosis page, you can use the "Forward" and "Backward" buttons to navigate through the entire process of the reduction division (another name for meiosis). You can either skip straight to the first meiosis and fertilization slide or you can view a diagram of the stages of meiosis* as an orientation. At the bottom of the page, there are a few links to sites which explain meiosis and may be able to help you if you are still confused.

Diagrammatic Meiosis*

This diagram of meiosis shows the cell undergoing the reduction division required to produce a haploid cell capable of fertilization (as, when the sperm - also haploid - fuses with the egg, the full chromosome number is restored. If this division didn't occur, when the egg and sperm fused, there would be too many chromosomes). When you are familiar with this, go to the first slide.

Diagramatic Meiosis
* This image taken from: http://www.gene.com/ae/AB/GG/meiosis1.html

To the first slide!

Sperm entrance Sperm Entrance

The non-flagellated haploid sperm (arrows) have entered the eggs. Meiosis can only occur after this has happened. Note the lack of extensive membranes which will be seen to surround the egg later.
Forward Backward First slide (Sperm entrance) Diagrammatic meiosis

First meiotic division 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 membranes which formed in association with fertilization (since not all of them have formed yet, they will not be labelled until later when they have all formed).
Forward Backward First slide (Sperm entrance) Diagrammatic meiosis

Second meiotic division 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.
Forward Backward First slide (Sperm entrance) Diagrammatic meiosis

Pronucleii fusion and end of meiosis
Completion of Meiosis and Joining of Pronuclei

The two Pronuclei are seen in the centre of the egg, touching one another, as they prepare to fuse to create a diploid cell (fusion of two haploid cells). This diploid cell restores the diploid number of chromosomes. Note the presence of two polar bodies now (arrows), 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 membranes are labelled as follows:

  1. The fertilization membrane (outer, darkly stained),
  2. The chitinous layer (middle, thick, moderately stained)
  3. The ascaroside layer (middle, very thin, pale staining) and
  4. The perivitelline space (inner, thick, pale staining)
Forward Backward First slide (Sperm entrance) Diagrammatic meiosis

Some Other Sites Explaining Meiosis

This page is part of the Developmental Biology ONLINE! web site.
Special thanks to Hans Christoffersen for the preparation of this page.

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Send comments to:

Dr. Steven Scadding ( scadding@uoguelph.ca ) or Sandra Frombach (sfrombac@uoguelph.ca),

Department of Zoology, University of Guelph, Guelph, Ontario, Canada N1G 2W1.

Last Revision: August 10, 1998