The correct answer is <span>hollow ball of cells formed after a fertilized cell undergoes cell division.
It is found in the earliest development of mammals and an embryo forms out of it.
</span><span>The definition of blastocyst
mammalian blastula in which some differentiation of cells has occurred.</span>
Acoelomates.
They lack a coelom (body cavity)
approximately 115 days
Human red blood cells (RBC), after differentiating from erythroblasts in the bone marrow, are released into the blood and survive in the circulation for approximately 115 days.