Embryology: General Principles and Comparative Development

A comprehensive set of practice questions covering general embryology, egg types, developmental stages, and comparative embryogenesis across insects, echinoderms, amphibians, birds, and humans.

What is the definition of embryology?

The study of an animal's development from the fertilized egg to the constitution of the organism.

What term describes the complete life history of an individual, starting from fertilization through old age?

Ontogeny

Which branch of embryology focuses on congenital anomalies and malformations due to developmental disturbances?

Pathological embryology or teratology

What are the four major stages of embryonic development in metazoans?

1. Period of premorphogenesis (fertilization and segmentation); 2. Period of primary morphogenesis (gastrulation); 3. Period of secondary morphogenesis (neurulation); 4. Period of definitive morphogenesis (organogenesis)

How do ovoviviparous organisms differ from viviparous organisms?

In ovoviviparity, the embryo develops in eggs kept inside the mother without a nutritive placental relation; in viviparity, the embryo develops in the uterus and feeds via a placenta.

What property of a zygote allows it to give rise to any cellular type or an entire organism?

Totipotency

Describe the polarity of a typical animal egg.

The egg has an animal pole ($AP$) which is poor in yolk and contains the nucleus, and a vegetal pole ($VP$) which is rich in yolk.

What is the chemical nature of yolk, and what are its two main components?

It is lipoproteic in nature, composed of proteins and phospholipids, often found in aggregates called yolk platelets.

Which type of egg is found in placental mammals and contains almost no yolk?

Alecithal eggs

What is the characteristic of centrolecithal eggs, and which animal group typically possesses them?

The yolk is concentrated in the center with cytoplasm as a thin peripheral layer; they are typical of insects.

How does holoblastic cleavage differ from meroblastic cleavage?

Holoblastic cleavage is the complete division of the egg (little yolk), while meroblastic cleavage is incomplete, occurring only in active cytoplasm (abundant yolk).

What are the three specific types of holoblastic cleavage based on blastomere arrangement?

Radial, spiral, and rotational.

What type of blastula is characterized by a central blastocoel surrounded by a single layer of blastoderm in oligolecithal eggs?

Regular coeloblastula

What is the primary outcome of gastrulation?

The transformation of the blastula into a three-layered embryo containing ectoderm, mesoderm, and endoderm.

Define the 'epiboly' movement during gastrulation.

The extension of a sheet of cells (usually micromeres) over deeper layers (macromeres) of the embryo.

What is the 'mid-blastula transition'?

The point when the embryonic genome becomes transcriptionally active, cell cycles lengthen, and $G1$ and $G2$ phases appear.

What type of cleavage occurs in the centrolecithal eggs of Drosophila?

Superficial meroblastic cleavage

In Drosophila, what does the mesoderm give rise to?

Heart, blood, circulatory system, muscles, endocrine glands, fat body, and gonads.

What is the name of the larval stage in sea urchin development?

Echinopluteus larva

In amphibians, what serves as the first sign of gastrulation?

The appearance of the blastoporal notch (dorsal lip of the blastopore) in the gray crescent region.

What is the function of 'bottle cells' in amphibian gastrulation?

They undergo invagination to form the dorsal lip of the blastopore and initiate the archenteron.

Which extra-embryonic membrane in birds and mammals directly encloses the embryo in a fluid-filled cavity?

Amnion

What are the functions of the allantois in bird embryos?

Respiratory gas exchange, digestive absorption of water/albumen, waste storage (uric acid), and skeletal mineralization.

In mammalian development, which two cell populations comprise the blastocyst?

The trophoblast (outer layer) and the inner cell mass (embryoblast).

What is the role of the syncytiotrophoblast during implantation in mammals?

It is a multinucleated mass that invades the uterine endometrium using proteolytic enzymes to allow the embryo to embed.

What structure in mammals and birds is the functional equivalent of the amphibian blastopore during gastrulation?

The primitive streak (specifically Hensen's node at its cranial end).