Which of the following is common to the development of both birds and mammals?
a. Holoblastic cleavage
b. Epiblast and hypoblast
c. Trophoblast
d. Gray crescent

Verified step by step guidance

1. Begin by understanding the terms related to embryonic development in birds and mammals. These terms are specific stages or structures that occur during the early development of an embryo.
2. Holoblastic cleavage refers to a type of cleavage where the entire egg is divided into smaller cells. This is common in species with little yolk, such as mammals. However, birds typically undergo meroblastic cleavage due to the presence of a large yolk.
3. The epiblast and hypoblast are layers formed during the early stages of embryonic development. The epiblast gives rise to the embryo itself, while the hypoblast contributes to the formation of extraembryonic structures. Both birds and mammals have these layers during development.
4. The trophoblast is a structure that forms part of the placenta in mammals, but it is not present in birds. It is involved in implantation and nutrient exchange between the mother and the developing embryo.
5. The gray crescent is a feature observed in amphibian development, particularly in frogs, and is not common to birds or mammals. It is involved in establishing the body axis during early development.

Key Concepts

Holoblastic Cleavage

Holoblastic cleavage refers to a type of embryonic development where the entire egg is divided into smaller cells. This process is common in species with eggs that have little to moderate yolk, such as mammals and some birds. It contrasts with meroblastic cleavage, where only a portion of the egg divides due to a large amount of yolk.

Epiblast and Hypoblast

The epiblast and hypoblast are two layers formed during early embryonic development in amniotes, including birds and mammals. The epiblast gives rise to the embryo proper, while the hypoblast contributes to the formation of extraembryonic structures. This bilaminar disc is crucial for the subsequent formation of the three germ layers through the process of gastrulation.

Trophoblast

The trophoblast is a layer of cells forming the outer shell of the blastocyst in mammals, playing a key role in implantation and placenta formation. It facilitates nutrient exchange between the mother and the developing embryo. While crucial in mammals, birds do not form a trophoblast, as their development occurs outside the maternal body in eggs.