Animal Developmental Patterns Practice Questions

Correct answer: Superficial cleavage

In most insects, the high concentration of yolk in the center of the egg restricts cleavage to the cytoplasmic rim. This results in a syncytial blastoderm stage where nuclei share a common cytoplasm before cellularization.

Correct answer: It serves as the site where gastrulation begins

The Gray Crescent forms opposite the point of sperm entry due to cortical rotation. This region contains cytoplasmic determinants that define the dorsal-ventral axis and initiate the formation of the blastopore lip.

Correct answer: The blastodisc

Because avian eggs are extremely telolecithal, cleavage is restricted to a small yolk-free disc of cytoplasm at the animal pole called the blastodisc. The surrounding yolk mass remains undivided and provides nutrients to the developing embryo throughout incubation.

Correct answer: Bicoid gene

Bicoid mRNA is anchored at the anterior pole of the egg by cytoskeletal proteins. After fertilization, the resulting Bicoid protein gradient activates target genes for head and thorax development, making it one of the best-characterized morphogens in developmental biology.

Correct answer: Inducing the neural tube and dorsal structures

The Spemann-Mangold Organizer, located at the dorsal lip of the blastopore, secretes inhibitory signals such as Chordin and Noggin that block BMP activity. This allows the overlying ectoderm to adopt a neural fate, forming the neural tube and establishing the dorsal body axis.

Correct answer: The blastopore

The primitive streak is the site of gastrulation in birds and mammals. Like the amphibian blastopore, it is the portal through which cells migrate to form the mesoderm and endoderm layers.

Correct answer: Egg, larva, pupa, adult

Holometabolous insects (complete metamorphosis) pass through four distinct stages: egg, larva, pupa, and adult. The pupal stage involves complete reorganization of larval tissues into the adult body plan, in contrast to hemimetabolous development which lacks a pupal stage.

Correct answer: The onset of zygotic transcription

During early cleavage, development is driven exclusively by stored maternal mRNA. At the mid-blastula transition, the cell cycle lengthens, and the embryo's own genome is transcribed for the first time, shifting developmental control from maternal to zygotic factors.

Correct answer: Dorsal lip of the blastopore

Hensen’s Node is located at the anterior end of the primitive streak. Like the dorsal lip (organizer) in amphibians, it acts as a signaling center that induces the primary body axis.

Correct answer: Allantois

The allantois collects nitrogenous waste (primarily uric acid) produced by the embryo. As it expands, it fuses with the chorion to form the chorioallantoic membrane, which lies close to the porous shell and serves as the primary respiratory surface for gas exchange.

Correct answer: Deletion of contiguous body segments

Gap genes such as Krüppel, Knirps, and giant define broad domains along the embryonic axis in response to maternal morphogen gradients. Loss-of-function mutations produce 'gaps' — the deletion of several consecutive body segments — leaving a discontinuous, truncated body plan.

Correct answer: Thyroxine (T4)

Thyroxine (T4) and its more active form T3 coordinate the sweeping morphological changes of amphibian metamorphosis, including tail resorption, limb growth, and restructuring of the digestive system. Without thyroid hormone signaling, a tadpole remains permanently in larval form.

Correct answer: Triggering molting and pupation

Ecdysone is the 'molting hormone' secreted by the prothoracic gland. When secreted in the presence of high juvenile hormone, it leads to a larval molt; in the absence of juvenile hormone, it leads to pupation.

Correct answer: Ectoderm

As cells ingress through the primitive streak, they displace the hypoblast to form endoderm or spread between layers to form mesoderm. The cells that do not migrate inward remain on the surface as the ectoderm.

Correct answer: Incomplete metamorphosis with nymph stages

Hemimetabolous insects such as grasshoppers, cockroaches, and dragonflies undergo incomplete metamorphosis. The immature stages, called nymphs, closely resemble wingless adults and gradually acquire adult features through successive molts, without passing through a discrete pupal stage.

Correct answer: Space between the blastoderm and the yolk

The subgerminal cavity is created when the blastoderm cells secrete fluid between themselves and the yolk. This space is essential for the eventual formation of the epiblast and hypoblast.

Correct answer: Determining the identity of specific segments

Hox genes specify the structures that grow on each segment (e.g., legs, wings, or antennae). Mutations in these genes, such as Antennapedia, can cause legs to grow where antennae should be.

Correct answer: The Spemann-Mangold Organizer

The Nieuwkoop Center is the dorsal-most vegetal region of the blastula. It produces signals that induce the overlying cells to become the Organizer, which then directs the rest of gastrulation.

Correct answer: The translucent central region of the blastoderm

The Area Pellucida is the central part of the avian blastoderm that appears clear because it is separated from the yolk by the subgerminal cavity. The surrounding region attached to the yolk is the Area Opaca.

Correct answer: Pair-rule genes

Pair-rule genes (like even-skipped) are expressed in seven transverse stripes. They act to divide the broad regions established by gap genes into the repeating units of the insect body plan.

Correct answer: Nuclear divisions occur repeatedly without forming new cell membranes

In Drosophila, the first 13 rounds of nuclear division occur without cytokinesis, creating a syncytium. This allows morphogens to diffuse freely across the embryo to establish the body axes.

Correct answer: Cortical rotation toward the sperm entry point

Cortical rotation occurs about 30 degrees opposite the site of sperm entry. This movement exposes the lightly pigmented inner cytoplasm, marking the future dorsal side of the embryo.

Correct answer: Chorioallantoic membrane

The chorioallantoic membrane (CAM) is formed by the fusion of the allantois and chorion. It functions in gas exchange and the resorption of calcium from the eggshell for bone mineralization.

Correct answer: Establishing anterior-posterior polarity within each segment

Segment polarity genes, such as engrailed and wingless, are activated after the pair-rule genes. They ensure that each segment has a distinct front and back end through local cell signaling.

Correct answer: Hensen's Node

Hensen's Node is located at the anterior end of the primitive streak. Like the dorsal lip of the blastopore in amphibians, it can induce a secondary body axis if transplanted to another embryo.

Correct answer: Induction of enzymes for the urea cycle

Tadpoles are ammonotelic (excrete ammonia), which is toxic but easily diluted in water. Adults become ureotelic, synthesizing urea via the urea cycle to conserve water and safely excrete nitrogen.

Correct answer: Suspend the yolk in the center of the albumin

Chalazae are twisted strands of protein (mucin) that anchor the yolk. They ensure the blastoderm remains positioned on top, regardless of how the egg is turned, to facilitate even warming during incubation.

Correct answer: Juvenile Hormone (JH)

Juvenile Hormone prevents metamorphosis. When JH levels are high, ecdysone triggers a molt from larva to larva; when JH levels drop, ecdysone triggers a molt to the pupal or adult stage.

Correct answer: Initiating blastopore lip formation by invagination

Bottle cells change shape dramatically, with their main body moving inward while remaining attached to the surface. This constricts the surface and initiates the inward folding that forms the blastopore.

Correct answer: All three germ layers of the embryo

In birds, the blastoderm splits into the epiblast and hypoblast. While the hypoblast forms extraembryonic structures, the epiblast contains all the cells that will form the ectoderm, mesoderm, and endoderm of the chick.

Correct answer: Nanos

Nanos mRNA is localized at the posterior pole of the egg. It acts by inhibiting the translation of Hunchback mRNA in the posterior, allowing posterior-specific genes to be expressed.

Correct answer: The development of wings during the nymphal stages

Hemimetabolous insects (like grasshoppers) undergo incomplete metamorphosis. The nymphs resemble the adults but lack functional wings and reproductive organs, which develop gradually through successive molts.

Correct answer: Small rapidly cleaving cells (micromeres)

The animal pole has less yolk, allowing cleavage furrows to pass through more easily and quickly. This results in the formation of smaller cells compared to the yolk-rich vegetal pole.

Correct answer: Primitive streak

The primitive streak is the defining feature of avian and mammalian gastrulation. It defines the major body axes and serves as the site where surface cells undergo an epithelial-to-mesenchymal transition to move inward.

Correct answer: Seven transverse stripes across the embryo

Pair-rule genes are expressed in 7 stripes, with each stripe corresponding to every other parasegment. They integrate the information from gap genes to begin the process of periodic patterning.

Correct answer: To act as a respiratory and excretory organ

The allantois is a sac-like structure that collects uric acid. It also fuses with the chorion to allow for the exchange of oxygen and carbon dioxide through the porous eggshell.

Correct answer: Apoptosis

Thyroid hormones trigger systematic apoptosis in specific tissues. For example, the cells of the tadpole tail are essentially instructed to die and are reabsorbed by the body to provide nutrients for adult structures.

Correct answer: Undifferentiated larval cells forming adult structures

Imaginal discs remain relatively quiescent during the larval stages. During the pupal stage, they undergo massive proliferation and differentiation to form the exterior structures of the adult insect.

Correct answer: Separating animal and vegetal cells before induction

The blastocoel is a fluid-filled cavity. By separating the cells of the animal cap from the vegetal cells below, it prevents the vegetal cells from inducing the animal cap cells to become mesoderm too early.

Correct answer: The dark peripheral ring attached to the yolk

The Area Opaca is the outer region of the blastoderm. It appears 'opaque' because its cells remain in contact with the yolk, unlike the Area Pellucida which is separated from the yolk by the subgerminal cavity.