Creating Bodies
by:Sruti Parvataneni, Cori Plesko, Madhu Prakash

An Introduction
In human development, several processes occur in order to create a new generation of organisms from the parent organisms. The formation of the new organism starts with one new cell, the fertilized egg, which further divides and specializes to create a body. After fertilization, in a nine month period, implantation, invagination, gastrulation, organ formation, neutralization, and cephalization need to happen in order for the fetus to be ready for birth.

What Leads Up to It
At the time of ovulation, the egg is released from the ovary. Fertilization of the egg by a sperm usually happens within 12-24 hours after ovulation in the distal part of the fallopian tube. The egg/sperm combination is now called a "zygote" and begins traveling down the Fallopian tube towards the uterus. During those 3 days that it spends in the fallopian tube, the fertilized egg divides into many cells called blasstomeres, undergoing its first set of mitotic divisions and first forming a solid mass of cells before it turns into a hollow ball of cells.
external image Blastocyst_English.svg
How a Blastocyst Divides to mature into its form right before Implantation
How a Blastocyst Divides to mature into its form right before Implantation

A Summary of Implantation
When this hollow mass of cells reaches the uterine cavity about 5-6 days after fertilization, it is called a blastocyst. It adheres to the lining of the uterus, called the endometrium, within 1-2 days (6-12 days after fertilization), establishing pregnancy.~5~
This would be days 20-24 on the menstruation cycle because ovulation typically occurs on day 14.
The outer layer of cells (trophoblast) on the blastocyst give rise to the placenta, umbilical cord, and other tissues needed for support of the future fetus when it attaches to the uterine lining, while the inner cell mass eventually forms the baby.

Development- From Oocyte stage to Embryonic Stage
Development- From Oocyte stage to Embryonic Stage

Stages of ImplantationThere are three stages of implantation: adplantation/apposition, adhesion, and invasion.
Representation of Adhesion and Invasion
Representation of Adhesion and Invasion

Once it is oriented correctly towards the endometrium (apposition), the blastocyst breaks out of its protective covering (Zona Pellucida). The endometrium is the lining of the uterus. The Zona Pellucida exists before apposition to prevent premature implantation.~6~

"Hatching" of the Blastocyst from the Protective Zona Pellucida
"Hatching" of the Blastocyst from the Protective Zona Pellucida

In this time while apposition is occurring, the endometrium changes shape and absorbs uterine fluid, creating a sort of vacuum to bring the blastocyst closer to its epithelial layer. Because the blastocyst is still not firmly embedded in the uterus, it can still be "flushed out"(~1~) and eliminated. Also, in this early stage, the blood circulation begins between mother and blastocyst, a process that will continue until birth.
Then, the inner cell mass adheres to the uterine wall (adhesion), with the help of an exchange of hormones and structures on the blastocyst, in a period of days called the "implantation window", about 6 days after the peak of the hormone LH in the body. This is usually the 20-23rd day of the menstrual cycle. In adhesion, the microvilli of the outermost trophoblast of the blastocyst interact with the glycoproteins on the epithelial layer of the endometrium to firmly anchor the blastocyst. To prevent miscarriage, the endometrium becomes thicker and the cervix is sealed by mucus. At this stage, the blastocyst can no longer be flushed out of the body system. Soon after adhesion, the trophoblast differentiates into two categories of cells: ST (syncytiotrophoblast) cells and CT (cytotrophoblast) cells. The ST cells cause lysis of endometrial cells, securing places for the endometrium to embed itself.
Adhesion, and Beginning of Differentiation of CT+ST Cells

Finally, the blastocyst penetrates the epithelial layer and invades the stroma (invasion). The stroma of the uterus is the soft tissue with many blood vessels that is mostly used for support. The ST Cells surround the embedded ball of cells to keep the maternal blood flowing into them: the blood comes from the new capillaries that grow in places where endometrial cells ruptured before. The outer layer of cells of the blastocyst are also called the epiblast, as they develop into the embryo and its amniotic cavity. The hypoblast, or the lower layer develop the yolk sac, which produces non-nucleated blood cells in the developing embryo. This would be about 13 days post-ovulation~3~.
Within three weeks, the blastocyst cells begin to grow as clumps of cells within that little ball, and the baby's first nerve cells have already formed. As a note: the developing baby is called an embryo from the moment of conception to the eighth week of pregnancy. After the eighth week and until the moment of birth, the developing baby is called a fetus.

Regulation of Implantation Overall, the molecular mechanisms of Implantation are not well understood and are on the cutting edge of research. What is understood is that many reactions involved are cascade reactions, so the presence of a small amount of an original hormone can cause many changes to simultaneously and rapidly occur. Growth factors, proteinases, adhesion molecules, female prostaglandins, cytokines, and hormones are needed as mediators to regulate all reactions. ~9~Many of the needed growth factors and hormones are secreted by endometrial glands and decidual cells ( connective tissue in the uterine mucous membrane), while the blastocyst secretes its own receptors in order for it to be able to anchor to the epithelial layers of the uterus.
Even before adplantation and apposition, the embryo and the endometrium communicate with each other near the uterine epithelium. After hatching, the surface proteins of the uterus, known as glycolax, decrease in density and there is a decrease in the electrostatic force between the blastocyst and endometrium: this decreases repulsion, aiding implantation. At this stage the blastocyst produces interleukin 1, which helps in orienting the blastocyst towards the endometrium. The platelet-activating factor (PAF) is also produced by the embryo to help plug the cervix and prevent miscarriage.

During adhesion, cadherins are calcium-dependent cell adhesion molecules that play a role in anchoring of the blastocyst in the endometrium. After adhesion, the attached blastocyst begins to secrete Human Chorionic Gonadotrophin (HCG). HCG is a glycoprotein and a heterodimer of the same 92 amino-acid alpha sub unit of DNA used by the hormones FSH, LH, and TSH.~7~ The term “heterodimer” is used to describe a complex formed when two different macromolecules bind to each other, usually non-covalently. HCG behaves like FSH and LH with one major exception: it is NOT inhibited by a rising level of progesterone. Thus, HCG enables pregnancy to continue beyond the end of the normal menstrual cycle.Because only the implanted trophoblast makes HCG, its early appearance in the urine of pregnant women provides the basis for pregnancy tests.
The hormones Progesterone and Oestrogen are thought to contribute to the blastocyst being pressed against the endometrium as adhesion occurs. As pregnancy continues, the placenta becomes a major source of progesterone (a female steroid hormone, nicknamed P4, which stimulates the uterus to prepare for implantation) , and its presence is essential to maintain pregnancy. Women may start menstruating if there is lack of progesterone because it maintains the blood-rich lining of the endometrium~2~, so miscarriage may occur in this way too. Furthermore, mothers at risk of giving birth too soon can be given a synthetic form of the pregnancy hormone progestin to help them retain the fetus until it is full-term. Also, elevated androgen and therefore, estrogen, levels are thought to have detrimental effects on the endometrium, and therefore, implantation.

As discussed in the Next Section, the Placenta is Essential to the development of the Embryo and Fetus: While having its own functions, it releases many hormones needed in implantation

Finally, during invasion, trophoblast cells secrete enzymes that allow the endometrium to become more porous for the invasion of the embryo. These enzymes are mainly matrix metalloproteinase (MMP) and plasminogen-activators.
Trophoblast cells of the blastocyst also express certain integrins (cell adhesion molecules) on their cell membranes: epiblast cells express integrins a5b1 and a1b1, which interact with the uterine mucous, while hypoblast cells create integrin a6 chains.
The growth of the trophoblast into the endometrium and the decomposition of the epithelia are controlled by endometrial factors (secreted by epithelial cells, fibroblasts, macrophages and leukocytes). These factors cause autocrine and paracrine effects in order to ease the invasion of the trophoblast into the uterine membranes. (Autocrine=Hormone affecting the same cell which produces it; Paracrine=Hormone's effects are localized)

If the zygote implants anywhere other than the inner lining of the uterus, it is called an extra-uterine (ectopic) pregnancy. 0.5-1% of all pregnancies are ectopic; most extra-uterine pregnancies occur in the fallopian tubes, but it is possible they may occur in the abdominal cavity, ovary, or cervix.Sometimes, the zygote implants in the lower part of the uterus, which causes the placenta to develop in the cervix: this type of implantation is called the placenta previa. Because the placenta would detach before the baby is born, a caesarian section is always performed when a placenta previa occurs. If the placenta were detached prematurely, hemorrhaging would occur, causing death of both baby and mother in many cases.~8~ It is clear that to improve successful implantation rates in humans, it is important to find ways to pinpoint the window of implantation, ensure that the best embryo is selected, and synchronize embryo transfer with the time of optimal endometrial receptivity.

An Illustration of the Various Viable Areas of Implantation of a Blastocyst
An Illustration of the Various Viable Areas of Implantation of a Blastocyst

Here's a nice video that provides an implantation summary ;)


Invagination: a sheet of cells (called an epithelial sheet) bends inward.Sea urchin gastrulation.Begins at the vegetal pole where individual cells enter the blastocoel as mesenchyme cells.The remaining cells flatten and buckle inwards: invagination-During invagination, an epithelial sheet bends inward to form an inpocketing. One way to think of this in three dimensions is to imagine that you are poking a partially deflated beach ball inward with your finger. The resulting bulge or tube is an invagination. If the apical side of the epithelium forms the lumen (central empty space) of the tube, then the movement is termed invagination. If the lumen is formed by basal surfaces, then the movement is termed an evagination.Cells rearrange to form the archenteron. Archenteron-vegetal plate that undergoes rearrangement of its. cell a process that transforms the shallow invagination into narrower pouchThe open end, the blastopore, will become the anus.An opening at the other end of the archenteron will form the mouth of the digestive tube. Invag Info
Sea Urchin Gastrulation
Sea Urchin Gastrulation


-gastrulation is a morphogenetic process
-gastrulation-dramatic rearrangement of the cells of the blastula

- gastrulation, cell movements result in a massive reorganization of the embryo from a simple spherical ball of cells, the blastula, into a multi-layered organism.
-During gastrulation, many of the cells at or near the surface of the embryo move to a new, more interior location.

-Gastrulation is when layers of embryonic tissues develops into adult body parts. The resulting development stage is called gastrula. (Book)
-The cellular mechanisms involved in gastrulation are common to all animals.
-The major types of cell movements that occur during gastrulation.
1)Invagination: a sheet of cells (called an epithelial sheet) bends inward.
2)Ingression: individual cells leave an epithelial sheet and become freely migrating mesenchyme cells.
3)Involution: an epithelial sheet rolls inward to form an underlying layer.
4)Epiboly: a sheet of cells spreads by thinning.
5)Intercalation: rows of cells move between one another, creating an array of cells that is longer (in one or more dimensions) but thinner.
6)Convergent Extension: rows of cells intercalate, but the intercalation is highly directional.

Embryonic stem cells are derived from the inner cell mass of the blastocyst. Embryonic stem cells in culture are capable of self-renewal without differentiation and are able to differentiate into all cell types of the endoderm, mesoderm and ectoderm lineages using appropriate signals.
-the blind pouch formed by gastrulation, called the Archenteron, opens to the outside via the blastopore.
-gastrulation rearranges the blastula to form a 3 layered embryo with a primitive gut.
-differs from one animal group to another, but common sets of cellular changes drives this spatial rearrangement of an embryo but the similarities are change in cell mortality, change in cell shape, change in cellular adhesion to other cells and to molecules of extracelluar matrix. GastrulationInfo

In utero, the blastocyst implants and all three embryonic germ layers are formed during gastrulation. Somatic stem cells are present in many fetal and post-natal tissues. Somatic stem cells are also capable of self-renewal and, with appropriate signals, differentiate into various cell types from the organ from which they are derived.

-the three cell layered embryo is called gastrula
-positioning of cell layers in the gastrula allows cells to interact with each other in new ways
-3 layers (also known as embryonic germ layers) called ectoderm,endoderm,mesoderm
1)ectoderm-forms the outer layer of gastrula
nervous system and epidermis of skin
2)endoderm-lines the embryonic digestive tract
innermost lining of digestive tract,liver,pancreas
3)mesoderm-fills space between ectoderm and endoderm
kidney,heart,muscle,and inner layer of skin(dermis)Gastrulation2Notes

.Frog gastrulation produces a triploblastic embryo with an archenteron.
-Where the gray crescent was located, invagination forms the dorsal lip of the blastopore.
-Cells on the dorsal surface roll over the edge of the dorsal lip and into the interior of the embryo:involution.
-As the process is completed the lip of the blastopore encircles a yolk plug.

Overview of Gastrulation
Overview of Gastrulation

Organ Formation
Organ Formation, or organogenesis, ususally starts right after gastrulation, or sometimes even gastrulation is underway. The germ layers, ectoderm, mesoderm, and endoderm begin subdividing into regions that will form various organs of the body. First, a mass of cells is set aside for an organ system; then, it is subdivided to form the single organ parts of that organ system. An organ is a collection of tissues joined together which perform a particular function. The initially formed part is called the primary organ rudiment, while the smaller parts are called the secondary organ rudiments. ~10~
The organization of the body of the organism is determined mostly in gastrulation, as groups of cells that were distant before start moving closer together, so there are chances of mixing of materials or binding to occur between the cells.
In the development of vertebrates, the sliding of cells (called the presumptive mesoderm) into the interior of the embryo and their placement on the dorsal (back) side of the archenteron (in the archenteric “roof”), in immediate contact with the overlying ectoderm, is important because at the start of gastrulation, ectoderm is incapable of progressive development of any kind. Only after invagination does the ectoderm acquire the ability development. The dorsal mesoderm of a vertebrate, which later differentiates into a notochord, prechordal mesoderm, and somites, causes the overlying ectoderm to differentiate as the neural plate. Lateral mesoderm causes the overlying ectoderm to differentiate as skin.
In embryonic induction, various parts of the embryo cause groups of cells to proceed along a particular path of development. The inducing substance of the mesoderm is most likely (research is still being done in this area) a protein or a nucleoprotein, which goes into the cytoplasms of reacting cells to change their genetic expression.
Induction is responsible not only for the subdivision of ectoderm into neural plate and epidermis but also for the development of a large number of organ rudiments in vertebrates. For example, the notochord is a source of induction for the development of the adjoining somites (parts of the mesoderm which will become skeletal muscle and dermis, or skin) and nephrotomes (section of mesoderm which eventually gives rise to the kidneys).

What Each Germ Layer Differentiates Into
What Type of Cells are Formed from the 3 Germ Layers
What Type of Cells are Formed from the 3 Germ Layers

Differentiation of Amniotic Embryo
Differentiation of Amniotic Embryo

Formation of the Heart

The rudiment of the heart in vertebrates develops from the ventral edges of the mesodermal mantle in the anterior part of the body. A group of mesodermal cells breaks away from the ventral edge, takes a position just underneath the pharyngeal endoderm, and becomes arranged in the form of a thin-walled tube, which will become the lining of the heart (called the endocardium). In reptiles, birds, and mammals, the embryo in early stages of development is flattened out on the surface of the yolk sac; therefore, what are morphologically the ventral edges of the mesodermal mantle are far apart. As a result of this arrangement, two endocardial tubes are formed, one on either side of the embryo. Then, when the embryo becomes separated from the yolk sac sometime after gastrulation, the two endocardial tubes meet the developing pharynx and fuse, producing a single heart rudiment. After the formation of the endocardium, the coelom in the lateral plate mesoderm adjoining the heart rudiment expands slightly and envelops the endocardial tube or tubes. The heart muscle layer, or myocardium, develops from the visceral layer of the lateral plate that is in contact with the endocardial tube; the parietal layer of the lateral plate forms the pericardium, or covering of the heart. The portion of the coelom surrounding the heart becomes separated from the rest of the body cavity and develops into the pericardial cavity (the fluid-filled space between the pericardium that lubricates the pericardial membranes). The endocardial tube branches anteriorly into two tubes, the ventral aortas.
The heart is initially a straight tube. However, it becomes twisted and subdivided into four main parts: the most posterior, the sinus venuos, the atrium, the ventricle, and the conus arterious. In the course of development in the more advanced vertebrates, the atrium and ventricle become partially or completely subdivided into right and left halves. In amphibians, only the atrium is separated into two halves. In reptiles, a partition separates the atria and part of the ventricle. In birds and mammals, the subdivision of the heart is complete, with two atria and two ventricles.
The complete subdivision of the heart is important for separating the lung blood supply from the general body circulation. Although, if this separation developed early in the embryo, it would create problems because the lungs of the embryo are not functional; the placenta enriches the embryo with oxygen in the blood. The partition between the atria in mammals remains incomplete, so that blood returning from the body and from the placenta enters into the right half of the heart but is shunted into the left half of the heart and thence again into general circulation. At birth, that area is closed by a membraneous flap, and oxygen-depleted blood from the body enters the right atrium, is channelled into the right ventricle, and to the lungs for oxygenating. From the paired forward extensions from the heart, the ventral aortas, loops develop between the pharyngeal clefts (derived from pharyngeal slits, which are formed in the 4th week). These are the aortic arches. The arches are laid down in all vertebrates: human embryos have six, although only 5 can be found after birth. The arches of the third pair develop as the carotid arteries, supplying blood to the head. Those of the fourth pair join to form the dorsal aorta, providing blood to most of the body. These are the called the systemic arches. The arches of the sixth pair are the pulmonary arches; in embryos they carry blood to the dorsal aorta, as well as to the lungs, but in fully developed amniotes (reptiles, birds, and mammals), they carry blood only to the lungs.

Fetal Heart Formation
Fetal Heart Formation

Neurulation contributes to two major creations in vertebrates during organogenesis.
1. Neurulation creates the neural tube that allows for the creation of the central nervous system. The neural tube forms the spinal cord and the brain.
2. Neurulation creates the neural crest, the group of cells that are separated during neural tube formation that moves away from the neural tube’s dorsal surface to create different types of cell sets. The neural crest assists in the creation of the peripheral nervous system as well as pigments and neurons. (5N)

Neurulation annimation
(1N) Watch a great video illustrating neurulation by clicking on this link above

Neurulation causes the formation of the neural tube. This is important because the neural tube is responsible for the formation of both the brain and the spinal cord. Neurulation also creates neural crest cells which create pigment cells and neurons along with other cell types after they move away from the neural tube.

(2N) Diagram of neurulation

The formation of a neural plate starts the process of Neurulation. A neural plate is formed when epithelial cells change their shape and thicken the ectoderm layer. This then causes the plate to fold due to adhesion from the sides of the cell. As the plate is folding, the edges meet at a midline to form a tube shape. The neural crest cells are the cells that lie at the tip of the fold that now lie between the tube and the epidermis. (2N)
This process is induced by the notochord, found in all chordate embryos, which regulates the location as well as the formation of the neural tube by controlling neural plate formation. (2N)
During the process of Neurulation stomites, which are blocks of cells that later on become vertebrae, ribs, muscles, or skin cells, form pairs in the neural tube. (2N)
After it develops, the neural tube closes along its whole length at different rates depending on the type of vertebrate it is. For amphibians, the neural tube closes all at once while in mammals the middle of the tube begins to close first and spreads to the ends. If the tube does not close correctly, problems can occur in the organism that can cause it to die before birth or to live a very short life after it is born. There are two stages of Neurulation: primary Neurulation and secondary Neurulation. (3N)

Diagram of neurulation


Primary Neurulation:
(2N, 3N, 5N)
This is the process that divides the ectoderm into three different parts, the neural tube, the epidermis and the neural crest cells.
Here, the neural plate folds to create the neural tube after the neural plate is formed. When the neural folds form from the edges of the neural plate, they form the epidermis that lies above the tube. This also creates a neural groove which is a groove that is formed between the folds of the embryo. The groove is important as is becomes bigger, eventually becoming a closed tube.
When the neural tube is formed notochord cells die off which is called apoptosis.
When the neural tube has closed, the neural crest cells differentiate and migrate into different cell types. If the neural plate is not formed properly then spina bifida can occur where the spinal cord will protrude out of the bones which can cause paralysis.

Secondary Neurulation:
In this stage of Neurulation, the ectoderm forms the medullary cord. The medullary chord is part of the lymph node with lymphatic tissues. The medullary cord forms another tube when it condenses and creates spaces that come together to form another tube. These tubes connect to the tubes formed in primary Neurulation and create a large neural tube that later becomes the brain and spinal cord.

Cephalization is the process of sensory organs and systems migrating to the anterior end of the organism. It is essentially the creation of a head in an organism as a separate region of the body. (2C)
Cephalization is the differentiation of an anterior end that is an evolutionary advancement. Cephalization also comes with the gathering of nervous tissue and feeding apparatuses in the head to join together different activities of the nervous system. Many organisms have specialized neural cells called photoreceptors at the end of their head that allow them to better sense the area around them. (3C)
Cephalization has been evolving along with complex nervous systems in vertebrates. There are many organs in their anterior end that help detect the environment including the eyes, ears, nose, and mouth. Each of these organs helps the organism by working together, responding to different stimulus, to allow the organism to best sense its environment. (2C)
Cephalization also has become more advanced with more advanced Neurulation. Neurulation creates the neural crest that assists in the creation of the peripheral nervous system as well as different anterior features such as jaws, teeth, larger brains, and facial bones. These are all adaptations that help organisms better understand their environment and give them better chances of survival by bring better equipped to find and hunt down prey. (3C)
Image on top showing organisms that do not have cephalization, lower image shows organism with cephalization.
Image showing sensory information centered in the head region(top two images from 1C, bottom image from 3C)


All of the following are events that occur in primary Neurulation EXCEPT:
a) Neural groove becomes larger
b) Neural crest cells differentiate
c) Neural crest cells migrate
d) Ectoderm forms medullary cord
e) Ectoderm divided into three parts

Which of the following organisms’ evolutionary history has not experienced cephalization?
a) Earthworms
b) Jellyfish
c) Humans
d) Fish
e) Dinosaurs

Neurulation helps in the creation of…
a) Spinal cord and brain
b) Digestive system
c) Digestive organs
d) Respiratory system
e) None of the above

Spina bifida occurs when which of the following does not form properly?
a) Spinal cord
b) Neural crest
c) Neural plate
d) Neural tube
e) Epidermis

What is invagination?
a.)a sheet of cells (called an epithelial sheet) bends inward.
b.) individual cells leave an epithelial sheet and become freely migrating mesenchyme cells.
c.) a sheet of cells spreads by thinning.
d.) rows of cells intercalate, but the intercalation is highly directional.
e.) b and c

In frog embryo gastrulation
a.)The open end, the blastopore, will become the anus
b.) produces a triploblastic embryo with an archenteron.
c.) the apical side of the epithelium forms the lumen (central empty space) of the tube
d.) invagination forms the dorsal lip of the blastopore.
e.) b and d
f.) c and d

What does the endoderm create?
a.) kidney
b.) liver
c.) nervous system
d.) heart
e.) a and d

Hatching occursa) When the blastocyst first finds an anchor in the endometriumb) After the decidual cells surrounding a blastocyst break down to start maternal blood flowc) In day 6 of ovulationd) In the process of Appositione) In the process of Adhesion
Which of the following is False About HCG?a) It is inhibited by progesterone and oestrogen.b) Only the trophoblast makes it in a blastocyst.c) It is a glycoprotein.d) It behaves much like FSH and LHe) None of the above
The Mesoderm is is differentiated into:a) lining of the digestive tractb) nervous systemc) epidermisd) the skeletone) the pituitary gland

AP Essay Question:
Explain the organ formation of the heart. Start with implantation and go through gastrulation and then talk about the organ formation

Sources for Neurulation:

(1N) link for animation
(2N) detailed information on the different aspects of neurulation as well as some diagrams
(3N)Campbell, Neil A., and Jane B. Reece. Biology. Sixth ed. Boston, MA: Pearson Custon/Benjamin Cummings, 2002. Print.
basic information on neurulation(4N)
(5N) more detailed information on neurulation and its different components

Sources for Cephalization:
(1C) diagrams and basic information
(2C) Campbell, Neil A., and Jane B. Reece. Biology. Sixth ed. Boston, MA: Pearson Custon/Benjamin Cummings, 2002. Print.
basic information
(3C) diagrams and more detailed information

Source for Invagination:
****-Definition of invagination and a diagram of it.

Sources for Gastrulation: -Diagram of Gastrulation and the process leading to the three germ layers. -Gastrulation definition in general and the Gastrulation process in the Grog and Sea Urchin

Sources for Implantation:

~5~ Summary of Implantation+Signs of Implantation
~6~ Summary of Normal/Abnormal Implantation
~1~ Description of Stages of Implantation
~3~ Adhesion/ Gastrulation Event Description+Timeline of Embryo Formation

~9~ Describes some sorts of Hormonal Regulation of Implantation~7~ Timeline of Organ Formation for Fetus ~2~ Talks about Apposition/ Provides Timeline of Pregnancy~8~ Website talks about Improper Implantation
Sources for Organ Formation:
~10~ Most information about organ formation found here ~11~Talks about the development of the endoderm

Book Source:
Campbell, Neil A., and Jane B. Reece. Biology. Sixth ed. Boston, MA: Pearson Custon/Benjamin Cummings, 2002. 637, 1005. Print.