The three primary germ layers are called ectoderm, mesoderm, and endoderm. once these three layers have formed, the developing trilaminar structure may be called a(

Gastrula

Explanation:

A hollow shaped embryonic structure formed after two weeks of fertilization with three layers is known as gastrula.

The gastrula is formed from the repeated division of the single-layered structured called blastula which later forms three layers called ectoderm, endoderm and mesoderm.

These layers during the process of gastrulation from different organs of the multicellular organisms like the endoderm forms the liver, the ectoderm form the nervous system, the skeletal bone and many others.

Since gastrula structure contains three dermal layers therefore gastrula is the correct answer.