This is a bryozoan embryo exhibiting its blastopore. These animals are discreet but ubiquitous in oceans and lakes all over the world.
What we see is the DNA inside the nucleus of the cells of the embryo. The color gradient indicates if the nuclei are closer (yellow) or further away (purple) from the microscope camera.
The embryonic cells are arranged in a circle and form a central opening that we call the blastopore. This opening, in bryozoans, will become the mouth of the animal after the embryo develops.
What about our mouth, where does it come from?
Live footage of entoprocts! Tiny colonial invertebrates that capture food with a crown of ciliated tentacles
A short video that I made about the embryonic development of the likeable Drosophila, also known as fruit fly or vinegar fly, won an honorable mention in the Small World in Motion.
The details on the techniques I used and the video on its full resolution are available for download and re-use on the Wikimedia Commons.