blastoderm, blastoderma

The thin, disk-shaped cell mass of a young embryo and its extraembryonic extensions over the surface of the yolk; when fully formed, all three primary germ layers (ectoderm, endoderm, and mesoderm) are present. SYN: germ membrane, germinal membrane, membrana germinativa. [blasto- + G. derma, skin]

- bilaminar b. the b. of a young embryo when it consists of only two of the three primary germ layers it will ultimately have.

- embryonic b. that part of the b. that takes part in the formation of the embryonic body.

- extraembryonic b. that part of the b. which is not incorporated in the embryo but forms membranes concerned in its nourishment and protection.

- trilaminar b. the b. after all three of the primary germ layers have been established.