What is the mass of stem cells produced in the first few days after fertilization called?

The correct answer is Morula stage.

Explanation:

Morula is the solid ball of cells formed by fertilized ovum and later forms blastula stage in the embryonic stage. This solid ball of the mass consists of 10 to 30 cells.  

It is the final stage before the formation of the blastocoel, a fluid-filled cavity. It is the initial embryonic stage where mammalian cells fate decided to become either internal or external.

Thus, the  correct answer is Morula stage.

primary succession occurs in essentially lifeless areas regions in which the soil is incapable of sustaining life as a result of such factors as lava flows, newly formed sand dunes, or rocks left from a retreating glacier. secondary succession occurs

trachea, alveoli, lung capillaries, arteries