Merriam-Webster Collegiate Dictionary - gastrula

 noun  (plural -las or ~e)  Etymology: New Latin, from gastr-  Date: 1876 an early metazoan embryo in which the ectoderm, mesoderm, and endoderm are established either by invagination of the blastula (as in fish and amphibians) to form a multilayered cellular cup with a blastopore opening into the archenteron or by differentiation of the blastodisc (as in reptiles, birds, and mammals) and inward cellular migration — compare blastula, morula  • ~r adjective