The outer layer of cells of morula absorbs nutrition from the uterine wall and is now referred to as trophoblast. The cells of trophoblast are flattened and the fluid absorbed by this trophoblast gets collected in the cavity called the blastocoel. At this stage the trophoblast becomes the blastocyst and this stage of development is known as the blastula.

The inner cell mass present in the blastocyst looks like a small knob at one pole and it is called the embryonal knob. The trophoblast (outer layer of blastomeres) does not take part in the formation of the embryo proper. It helps in the formation of the extraembryonic membranes. The cells of the trophoblast that remain on contact with the embryonal knob are called cells of Rauber.