<< Chapter < Page Chapter >> Page >
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.

Got questions? Get instant answers now!

Questions & Answers

What is biology?
khan Reply
what element in colors purple
Mikaela Reply
what are the function of sympathetic nervous system and parasympathetic nervous system
Ubon Reply
what is preganglionic
Ubon
don't know
Sweety
what is reproduction
Aben
reproducing specifically : the process by which plants and animals give rise to offspring and which fundamentally consists of the segregation of a portion of the parental body by a sexual or an asexual process and its subsequent growth and differentiation into a new individual.
Black
is the process where by organism produce their new organism of the same species from those who a ready in existence.
Prudent
what is unicellular cell
Abigail
unicellular orgnism* meaning consisting of a single cell
Black
How does reproduction take place in human being
Aben
Is a single celled organism
Brian
What is the Antibiotic
Tamara Reply
antibiotics is any medication that stop the growth of bacteria
onuoha
what is biomolecules
Lawerence Reply
This is a group of molecules produced by a living organism
Odion
thank alot,I had a hard time getting the answer
Lawerence
wat is the meaning of Mr.niger. d
It Reply
Now Mr. Niger Dac M;movement R;respiration N;nutrition G;growth E;excretion R;reproduction D;death A;adaptation C;competition
Odion
Sorry.... I;irritability
Odion
GOOD MORNING MY NAME IS MESUMBE PRECIOUS WHAT IS YOUR NAME
Mesumbe Reply
ccc
Peace
hi
Emmanuel
Emmanuel Daniel
Emmanuel
what is the meaning of Mr Niger?
Emmanuel
the meaning of Mr Niger, movement , reproduction , nutrition, inspiration , growth, excretion ,reproduction
Shonde
Emmanuel Daniel it's Mr NIGER D
debby
and d is for death
Shonde
Brown Jones ug
Anguyo
state all the elements and their symbols
Taiwo Reply
what types of muscles are found in the heart
Diamond Reply
Cardiac muscle
Divya
cardiac or myocardia muscle
onuoha
cardiac muscles
Elvis
cardiac muscle
Jemima
Cardiac muscle
Dr
Thank you dear!
yimam
please can you tell me the meaning of Mr Niger?
Emmanuel
It is an initial to represent life processes of organisms. M means movement, R for reproduction, N stands for nutrition, I for irritability or sensitivity,G means growth , E for excretion and R stands for respiration.
Quartey
cardiac muscle
Stanisla
what is digestion
Dolla Reply
it is the chemical break down of insoluble food substances such as fatty acid to soluble substances which are then used for body processes
Lawerence
does human being sperm performed in the ovary of goat
sadeeq Reply
levels of ecological study
sadeeq
no
Dolla
human sperms contains acid
Dolla
what is an artery
Dolla
It is one of the organs of the circulatory system that carries oxygenated blood from the heart to other parts of the body.
Quartey
artery is type of blood vessel which carries blood away from heart...
shams
how liver destroy red blood cells?
shams
big bang theory was discovered by
Sweety Reply
Georges Lemaître
Mr
the meaning of nucleus
Spring Reply
nucleus Is also known as the cell's headquarters
Feranmi
what is pathology?
Azi Reply
is the study of diseases, it's cause and development of a disease
Boadi
State the law of segregation
Theolla Reply

Get the best Biology course in your pocket!





Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Biology' conversation and receive update notifications?

Ask