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.
You can follow the process on video or learn more details in the paper.
What about our mouth, where does it come from?