At the beginning of gastrulation, how many cells are generally present in an embryo?

• A. Between 4 and 6
• B. Between 6 and 40
• C. Between 40 and 10,000
• D. Between 10,000 and 100,000

Answer: (C)

At the beginning of gastrulation, the embryo is typically in the blastula stage, which consists of a spherical layer of cells formed from repeated cell division following fertilization. During this early development, particularly in many vertebrates, the number of cells can range from around 40 to several thousand, which aligns with the choice indicating a range between 40 and 10,000 cells. This phase involves the transition from a single layer of cells to multiple layers, marking a crucial point in embryonic development where the three primary germ layers—ectoderm, mesoderm, and endoderm—begin to form.

During this time, cellular arrangement and differentiation processes are critical as they set the foundation for the complex structures and systems that will develop in the organism. Prior to gastrulation, the embryo undergoes cleavage, where it rapidly divides without significant growth, resulting in an increasing number of smaller cells, eventually leading to the blastula stage with a more substantial total cell count conducive to gastrulation.