Animal Developmental Biology Questions and Answers

Answer: B

Digit pattern is set by Sonic hedgehog.

Sonic hedgehog is secreted by the zone of polarising activity at the posterior margin of the limb bud. Its concentration and the length of exposure specify digit identity along the anterior-posterior axis: grafting extra Shh-producing cells to the anterior margin produces mirror-image extra digits.

Desert hedgehog acts in the testis and peripheral nerves, and Indian hedgehog in cartilage and bone growth. Members of the TGF-β family such as BMPs act later in the limb, for instance in removing tissue between the digits, but they are not the morphogen that patterns digit identity.

Answer: D

The process is invagination.

Drosophila gastrulation begins on the ventral side, where presumptive mesoderm cells constrict at their apical surfaces. The change in cell shape bends the epithelial sheet inward into a ventral furrow, which folds into the embryo: invagination.

Involution is the rolling of a sheet inward over a lip, as at the blastopore of amphibians. Convergent extension narrows and lengthens a tissue by cells intercalating, as in germ band extension. Epiboly is the spreading of an epithelial sheet over the embryo's surface.