Question

A student encounters an animal embryo at the eight-cell stage. The four smaller cells that comprise one hemisphere of the embryo seem to be rotated 45 degrees and lie in the grooves between larger, underlying cells (spiral cleavage).

A. upon gastrulation, a solid ball of cells will be produced.
B. upon metamorphosis, the resulting trochophore larva will gain a backbone.
C. all of the answers are correct.
D. the first opening of the gastrula will ultimately serve as the mouth.

Answer 1

The first opening of the gastrula is called the blastopore, and for the organism in question, which exhibits spiral cleavage, this opening is destined to become the mouth. The correct answer to the student's question is option D.

Step-by-step explanation:

Gastrulation in Embryonic Development

The scenario described presents an animal embryo with spiral cleavage, which is indicative of certain invertebrates, and suggests that the organism will undergo gastrulation. During gastrulation, the blastula rearranges to form multiple layers that differentiate into various organ systems. The correct answer to this question is D, which states that the first opening of the gastrula, known as the blastopore, will ultimately serve as the mouth. This is typical in protostome development seen in many invertebrate species.

Options A and B are not correct because a solid ball of cells is not formed upon gastrulation, and the trochophore larva does not gain a backbone during metamorphosis, as the organism in question seems to belong to an invertebrate phylum. The blastula is often a hollow ball of cells at one stage in development, and the three germ layers formed during gastrulation are the endoderm, mesoderm, and ectoderm, each responsible for different tissues and organ systems.