This is a bryozoan embryo with its blastopore facing us. The blastopore is the hole in the middle. The “balls” are the nuclei of the cells that make up the embryo.
While cells have many other structures, they are not visible in this image. The only thing visible is the DNA because I stained it with a fluorescent dye (DAPI).
The colors aren’t their true color, though. But they indicate the 3D depth of each nucleus in the image, from yellow (what’s in the front) to purple (what’s in the back).
The front nuclei are bordering the blastopore forming a neatly arranged circle around this hole. So, that’s this opening anyway?
In bryozoans, the blastopore opening forms during gastrulation and will become the mouth of the animal. This is something I discovered by tracking each individual embryonic cell over time from egg to larva.
But please note that the blastopore doesn’t become the mouth in every animal. In some, including us, the blastopore becomes something else. Can you guess?