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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

differences between Homo sapiens and other primates
Aphiwe Reply
what is cell?
V.S.Nikhil Reply
The smallest structure and functional unit
Hydra reproduce through which process
Saint Reply
which is smallest organ in our body
pineal gland
Yh in the ears...
why you hand plam is sweating in everytime
who is the father of mycology
Sagar Reply
Heinrich Anton de Bary
describe the similarities and differences between cytokinesis mechanism found in animal cells versus in plant cells
hiro Reply
what is life?
life is the existantce of indvidual human or animal.
are humans beings considered to have the eukaryotic cells
success Reply
eukaryotes are organisms that possess cells with a nucleus enclosed in a membrane, humans, and all complex organisms are eukaryotes.
so humans and animals also have cell membranes.... cause I did this test prep and they said plants...I just want to be sure
and thank you for your reply it was helpful👍✌
eu= "perfect", "good", karyon= nut, amound, nucleus
you're welcome. Plants are also eukaryotes.
plants, like animals, possess a nucleus bound by a membrane.
similarities and differences between cytokinesis mechanism found in animal cell vs cell division
Raymark Reply
what is the name of a male flower?
Ikeomu Reply
staminate means flower containing only stamen
what is the definition of evolution in a population?
Homero Reply
the slow changing of a species to adapt to any changes in the environment or how it feeds/hunts. im not good at explaining things lol.
the organ which is sensitive to light in euglena
Fatimah Reply
the organ which is sensitive to light in euglena is
all chlorophyll containing motile cells are sensitive to light
there is no more other chapter
Sandeep Reply
Give tow examples for nutritional deficiency Diseases-
Singampalli Reply
How does a plant cell look like
Sang Reply
in a sleepers form
what do you mean ? I could not understand
they have a regular shape and a large vacoule
I thought it looked like rectangle
a stage in mitosis wherein in spindle fibers begin to shorten to pu the sister chromatids away from each other towards the opposite ends of the cell
Earl Reply
a stage in interphase where chromosome s are duplicated
What is biodiversity
Sp Reply

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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