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Illustration A shows the egg, nymph and adult stages of a grasshopper. The nymph stages are similar in appearance to the adult stage, but smaller. Illustration B shows the egg, larvae, pupa and adult stages of a butterfly. The pupa is a cocoon the butterfly makes when transforming from the larval to adult stages. The winged adult butterfly looks nothing like the caterpillar larva.
(a) The grasshopper undergoes incomplete metamorphosis. (b) The butterfly undergoes complete metamorphosis. (credit: S.E. Snodgrass, USDA)

The process of animal development begins with the cleavage    , or series of mitotic cell divisions, of the zygote ( [link] ). Three cell divisions transform the single-celled zygote into an eight-celled structure. After further cell division and rearrangement of existing cells, a 6–32-celled hollow structure called a blastula    is formed. Next, the blastula undergoes further cell division and cellular rearrangement during a process called gastrulation. This leads to the formation of the next developmental stage, the gastrula    , in which the future digestive cavity is formed. Different cell layers (called germ layers ) are formed during gastrulation. These germ layers are programmed to develop into certain tissue types, organs, and organ systems during a process called organogenesis    .

The left part of the illustration shows a single-celled zygote. The initial cleavage, or cell division, results in a ball of cells, called the eight-cell stage. The cells do not grow during cleavage, so the eight-cell stage ball is about the same diameter as the zygote. Further cleavage results in a hollow ball of cells called a blastula. Upon gastrulation, part of the ball of cells invaginates, forming a cavity called a blastopore.
During embryonic development, the zygote undergoes a series of mitotic cell divisions, or cleavages, to form an eight-cell stage, then a hollow blastula. During a process called gastrulation, the blastula folds inward to form a cavity in the gastrula.

Watch the following video to see how human embryonic development (after the blastula and gastrula stages of development) reflects evolution.

The role of homeobox ( Hox ) genes in animal development

Since the early 19 th century, scientists have observed that many animals, from the very simple to the complex, shared similar embryonic morphology and development. Surprisingly, a human embryo and a frog embryo, at a certain stage of embryonic development, look remarkably alike. For a long time, scientists did not understand why so many animal species looked similar during embryonic development but were very different as adults. They wondered what dictated the developmental direction that a fly, mouse, frog, or human embryo would take. Near the end of the 20 th century, a particular class of genes was discovered that had this very job. These genes that determine animal structure are called “homeotic genes,” and they contain DNA sequences called homeoboxes. The animal genes containing homeobox sequences are specifically referred to as Hox genes . This family of genes is responsible for determining the general body plan, such as the number of body segments of an animal, the number and placement of appendages, and animal head-tail directionality. The first Hox genes to be sequenced were those from the fruit fly ( Drosophila melanogaster ). A single Hox mutation in the fruit fly can result in an extra pair of wings or even appendages growing from the “wrong” body part.

While there are a great many genes that play roles in the morphological development of an animal, what makes Hox genes so powerful is that they serve as master control genes that can turn on or off large numbers of other genes. Hox genes do this by coding transcription factors that control the expression of numerous other genes. Hox genes are homologous in the animal kingdom, that is, the genetic sequences of Hox genes and their positions on chromosomes are remarkably similar across most animals because of their presence in a common ancestor, from worms to flies, mice, and humans ( [link] ). One of the contributions to increased animal body complexity is that Hox genes have undergone at least two duplication events during animal evolution, with the additional genes allowing for more complex body types to evolve.

Art connection

This illustration shows the four clusters of Hox genes found in vertebrates: Hox-A, Hox-B, Hox-C, and Hox-D. There are 13 Hox genes, but not all of them are found in each cluster. In  both mice and humans, genes 1–4 regulate the development of the head. Genes 5 and 6 regulate the development of the neck. Genes 7 and 8 regulate the development of the torso, and genes 9–13 regulate the development of the arms and legs.
Hox genes are highly conserved genes encoding transcription factors that determine the course of embryonic development in animals. In vertebrates, the genes have been duplicated into four clusters: Hox-A , Hox-B , Hox-C , and Hox-D . Genes within these clusters are expressed in certain body segments at certain stages of development. Shown here is the homology between Hox genes in mice and humans. Note how Hox gene expression, as indicated with orange, pink, blue and green shading, occurs in the same body segments in both the mouse and the human.

If a Hox 13 gene in a mouse was replaced with a Hox 1 gene, how might this alter animal development?

Section summary

Animals constitute an incredibly diverse kingdom of organisms. Although animals range in complexity from simple sea sponges to human beings, most members of the animal kingdom share certain features. Animals are eukaryotic, multicellular, heterotrophic organisms that ingest their food and usually develop into motile creatures with a fixed body plan. A major characteristic unique to the animal kingdom is the presence of differentiated tissues, such as nerve, muscle, and connective tissues, which are specialized to perform specific functions. Most animals undergo sexual reproduction, leading to a series of developmental embryonic stages that are relatively similar across the animal kingdom. A class of transcriptional control genes called Hox genes directs the organization of the major animal body plans, and these genes are strongly homologous across the animal kingdom.

Art connections

[link] If a Hox 13 gene in a mouse was replaced with a Hox 1 gene, how might this alter animal development?

[link] The animal might develop two heads and no tail.

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Questions & Answers

can biology be also be define has a scientific study that deal with life?
Stanley Reply
do you care asking you a question
Afolayan Reply
name the gas that diffuses into the plants leaves on a bright, sunny day
Tuhemwe Reply
explain why this gas diffuses into the plant's leaves
name two gases that diffuses out of the plant's leaves on a bright sunny day
Biology olympaid
what is ecosystem
Rondy Reply
why does a car move yet it is not a living thing?
this is because of the engine and fuel placed on
I also think it because force has been apply to it.
what is the reason of swelling due to fracture
Ejaz Reply
Sngy alaka
how do animals reproduce
Kelvin Reply
be specific because there are so many types of animals and how they reproduce are different
just be specific with a particular animal
what happens when you sneeze
Asuquo Reply
The sneeze center sends out a signal to tightly close your throat, eyes and mouth. Your chest muscles contract and compress your lungs while your throat muscles relax. All of that means air, saliva and mucus is forced out of your nose and mouth. AAAAAHHHH-CHOOOO.
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I have a question plzz
what is globsl warming
Global warming is the overall rise in temperature of the Earth itself which is caused by multiple factors. Such factors include: greenhouse gases, the burning of fossil fuels and other forms of human activity.
It's overall effects are known as climate change.
No problem.
Thank you.
what is creatinine?
Creatinine is a waste product produced in the body during muscle metabolism. This waste product is expelled from the body through urine. Here is it's formula:  C4H7N3O
Formula: C4H7N3O
Outline the process of cell in the body interms of it's function.
Aliruku Reply
what communication method used mainly by plants
Jerda Reply
what is biology
james Reply
Biology is the study of life
define nerve impulse
Karlin Reply
A nerve impulse is the way nerve cells (neurons) communicate with one another. Nerve impulsesare mostly electrical signals along the dendrites to produce a nerve impulse or action potential. The action potential is the result of ions moving in and out of the cell.
What is the Molecular Biology?
Service Reply
The branch of biology that deals with the structure and function of the macromolecules (e.g. proteins and nucleic acids) essential to life.
what is botany
Description Description Botany, also called plant science, plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field.
what is the difference between mitosis and meiosis?
Fatima Reply
what is an element
Saidu Reply
Element is any one of the simplest chemical substances that cannot be decomposed in a chemical reaction.
an element is a substance that cannot be broken down into any other substance
explain why only one sperm fertilisers the ovum
Frazzy Reply
its because other sperms are destroyed on the way
A test tube is full of a colourless gas that puts a lighted wooden splint.what gas is this?

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