The ball of cells is referred to as a blastula, once cleavage has produced around 100 cells. In most mammals, including humans, the structure formed next is the blastocyst, a mass of inner cells that are distinct from the blastula.

seven types

In mammals, the blastula forms the blastocyst in the next stage of development. Here the cells in the blastula arrange themselves in two layers: the inner cell mass, and an outer layer called the trophoblast. The inner cell mass is also known as the embryoblast and this mass of cells will go on to form the embryo.

BLASTULA PERIOD (2 1/4 – 5 1/4 h) We use the term blastula to refer to the period when the blastodisc begins to look ball-like, at the 128-cell stage, or 8th zygotic cell cycle, and until the time of onset of gastrulation, ca. cycle 14.

The cells of the blastula form an epithelial (covering) layer, called the blastoderm, enclosing a fluid-filled cavity, the blastocoel and are known as a blastocyst in mammals. So the correct option is ‘Blastocyst’.

Embryonic development begins with a sperm fertilizing an egg cell to become a zygote, which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoel is formed does the early embryo become a blastula….Blastulation.

So, the correct answer that describes a blastula is ‘A hollow ball of cells with a fluid-filled cavity called a blastocoel’.

Blastocyst, a distinctive stage of a mammalian embryo. It is a form of blastula that develops from a berrylike cluster of cells, the morula. A cavity appears in the morula between the cells of the inner cell mass and the enveloping layer. This cavity becomes filled with fluid.

The blastula is usually a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-filled cavity (the blastocoel). Mammals at this stage form a structure called the blastocyst, characterized by an inner cell mass that is distinct from the surrounding blastula.

rotational cleavage

In mammals humans, the rate and pattern of cleavage are non-specific. Complete answer: The cleavage in which the egg is divided unequally is called an unequal cleavage. Humans and Marsupials are an example of unequal holoblastic cleavage.

Definition of Cleavage. It is the repeated mitotic division of the zygote resulting in an increasing number of cells. The daughter cells generated by these divisions are termed blastomeres. The process begins soon after fertilization and continues as the zygote moves down the uterine tube (diagram)

Holoblastic, unequal:- the cleavage in which segmentation line divides egg into two unequal halves. This does not occur in mammals. So this option is correct.

The human ovum consists of protoplasm that contains some yolk, enclosed by a cell wall consisting of two layers, an outer one (zona pellucida) and an inner, thin one (vitelline membrane). isolecithal ovum one with a small amount of yolk evenly distributed throughout the cytoplasm.

uniformly distributed yolk are termed isolecithal. Eggs with abundant yolk concentrated in one hemisphere of the egg are termed telolecithal. This occurs in many invertebrates and in all vertebrates lower than marsupial mammals.

: characterized by complete cleavage that divides the whole egg into distinct and separate blastomeres — compare meroblastic.

The frog’s egg is telolecithal, meaning that there is a large amount of yolk concentrated at one pole, opposite to the concentration of cytoplasm and the location of the nucleus. The cleavages are holoblastic (i.e., total), and, after the second cleavage, they are unequal.

holoblastic type

Holoblastic cleavage – is a complete cleavage meaning it completely penetrates the egg. Meroblastic cleavage – is incomplete or partial cleavage meaning it does not penetrate the egg completely. The answer is yolk content of the egg. Because yolk is bulky, it is difficult for the cleavage to take place.

(embryology) The incomplete cleavage in telolecithal or megalecithal eggs of animals, such as birds and reptiles. Depending mostly on the amount of yolk in the egg, cleavage can be holoblastic (complete or total) or meroblastic (partial). In meroblastic cleavage, only a portion of the egg divides.

During cleavage, the cells divide without an increase in mass; that is, one large single-celled zygote divides into multiple smaller cells. The inner cell mass is also known as the embryoblast; this mass of cells will go on to form the embryo.

In telolecithal eggs, the yolk is unevenly distributed in the ovum cytoplasm. In centrolecithal eggs, the yolk is concentrated in the center of the cytoplasm of the ovum cytoplasm. Throughout the cytoplasm of the ovum, along the vegetal pole egg yolk is distributed.

->The polylecithal eggs are found in prototherian mammals, reptiles, birds. It is a characteristic of birds and reptiles. ->Centrolecithal egg- egg of many arthropods and some coelenterates are called centrolecithal. They are relatively large and have a great amount of yolk.

Centrolecithal (Greek kentron = center of a circle, lekithos = yolk) describes the placement of the yolk in the centre of the cytoplasm of ova. Many arthropod eggs are centrolecithal. This is due to the large dense yolk found within centrolecithal eggs and triggers a delayed embryonic development.

The key difference between the Telolecithal and Centrolecithal eggs is based on the distribution of the yolk in the cytoplasm of the ovum. In telolecithal egg, the yolk is unevenly distributed in the ovum cytoplasm. In centrolecithal egg, the yolk is concentrated to the center of the ovum cytoplasm.

Yolk becomes more abundant and tends to concentrate in one hemisphere of the egg. Because of the uneven distribution of yolk, such an egg is said to have a vegetal pole, where the concentration of yolk is greatest and an animal pole, where the concentration of yolk is smallest.

Birds have a telolecithal type of egg with a large and dense yolk spread throughout most of the egg, but separate from the pole of the developing embryo. The embryonic development in birds occurs in the blastodisc (also called germinal disc or germinal layer).

Telolecithal eggs are characteristic of birds, fishes, and reptiles while centrolecithal eggs are characteristic of insects.