It is essentially a hollow sphere that is composed of a shell - like outer cell layer and a small cluster of about 30 cells called the inner cell mass, which is attached to the inside wall of the sphere.
See kujutab endast seest tühja kera, mis koosneb kestataolisest välimisest rakkude kihist ja kera sisepinnale kinnitunud umbes 30 rakust koosnevast väikesest kobarast ehk siserakumassist.