Which Animal Is Most Like Humans During the Fetal Stage

Process by which the embryo forms and develops

Diagram of stages of embryo development to a larval and adult phase.

In developmental biology, animal embryonic development, also known as embryogenesis, is the developmental stage of an fauna embryo. Embryonic evolution starts with the fertilization of an egg cell (ovum) by a sperm cell, (spermatozoon).^[1] In one case fertilized, the ovum becomes a unmarried diploid cell known equally a zygote. The zygote undergoes mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of a multicellular embryo^[2] after passing through an organizational checkpoint during mid-embryogenesis.^[3] In mammals, the term refers chiefly to the early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.^[2] ^[four]

The main stages of beast embryonic evolution are every bit follows:

The zygote undergoes a series of prison cell divisions (called cleavage) to form a construction chosen a morula.
The morula develops into a structure called a blastula through a process chosen blastulation.
The blastula develops into a construction called a gastrula through a process called gastrulation.
The gastrula then undergoes farther development, including the formation of organs (organogenesis).

The embryo and then transforms into the next stage of development, the nature of which varies between different animal species (examples of possible next stages include a fetus and a larva).

Fertilization and the zygote [ edit ]

The egg cell is generally asymmetric, having an animate being pole (future ectoderm). It is covered with protective envelopes, with different layers. The first envelope – the i in contact with the membrane of the egg – is made of glycoproteins and is known as the vitelline membrane (zona pellucida in mammals). Different taxa testify unlike cellular and acellular envelopes englobing the vitelline membrane.^[2] ^[5]

Fertilization is the fusion of gametes to produce a new organism. In animals, the procedure involves a sperm fusing with an ovum, which somewhen leads to the development of an embryo. Depending on the beast species, the process tin can occur inside the torso of the female in internal fertilization, or exterior in the case of external fertilization. The fertilized egg jail cell is known as the zygote.^[2] ^[6]

To prevent more than than i sperm fertilizing the egg (polyspermy), fast block and tiresome block to polyspermy are used. Fast cake, the membrane potential rapidly depolarizing and then returning to normal, happens immediately later an egg is fertilized by a single sperm. Dull block begins in the beginning few seconds after fertilization and is when the release of calcium causes the cortical reaction, in which diverse enzymes are released from cortical granules in the eggs plasma membrane, causing the expansion and hardening of the outside membrane, preventing more than sperm from entering.^[7] ^[viii]

Cleavage and morula [ edit ]

Cell divisions (cleavage)

Cell division with no significant growth, producing a cluster of cells that is the aforementioned size as the original zygote, is chosen cleavage. At least four initial cell divisions occur, resulting in a dense ball of at least xvi cells called the morula. The different cells derived from cleavage, upwards to the blastula phase, are called blastomeres. Depending mostly on the amount of yolk in the egg, the cleavage can exist holoblastic (total) or meroblastic (partial).^[9] ^[10]

Holoblastic cleavage occurs in animals with lilliputian yolk in their eggs,^[11] such as humans and other mammals who receive nourishment as embryos from the female parent, via the placenta or milk, such as might be secreted from a marsupium. Meroblastic cleavage occurs in animals whose eggs take more yolk (i.eastward. birds and reptiles). Because cleavage is impeded in the vegetal pole, there is an uneven distribution and size of cells, beingness more than numerous and smaller at the animal pole of the zygote.^[9] ^[ten]

In holoblastic eggs, the first cleavage always occurs forth the vegetal-animate being centrality of the egg, and the 2nd cleavage is perpendicular to the start. From here the spatial organisation of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms:

**Cleavage patterns followed past holoblastic and meroblastic eggs in animals**
Holoblastic	Meroblastic
Radial (sea urchin,^[10] ^[12] amphioxus) Bilateral (tunicates,^[10] ^[thirteen] amphibians ^[fourteen]) Spiral (annelids, mollusks ^[x] ^[fifteen]) Rotational (placental mammals, marsupials,^[x] ^[16] nematodes ^[17])	Discoidal (fish, monotremes, birds,^[ten] ^[16] reptiles ^[10]) Superficial (insects ^[10] ^[18])

The finish of cleavage is known as midblastula transition and coincides with the onset of zygotic transcription.

In amniotes, the cells of the morula are at first closely aggregated, but soon they become bundled into an outer or peripheral layer, the trophoblast, which does non contribute to the germination of the embryo proper, and an inner jail cell mass, from which the embryo is developed. Fluid collects between the trophoblast and the greater part of the inner cell-mass, and thus the morula is converted into a vesicle, chosen the blastodermic vesicle. The inner prison cell mass remains in contact, yet, with the trophoblast at one pole of the ovum; this is named the embryonic pole, since it indicates the location where the future embryo will develop.^[nineteen] ^[x]

Germination of the blastula [ edit ]

After the seventh cleavage has produced 128 cells, the morula becomes a blastula.^[nine] The blastula is usually a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-filled cavity (the blastocoel or blastocyst).^[20] ^[21] ^[22]

Mammals at this stage form a structure called the blastocyst,^{atOptions = {
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Which Animal Is Most Like Humans During the Fetal Stage