b.What are the three primary germ layers that form during embryonic development

ECTODERM: skin, nervous system, teeth
MESODERM: bones, muscles, reproductive system, circulatory system, heart, kidneys, connective tissue
ENDODERM: lungs, digestive tract, liver, pancreas, bladder

Germ layers arise after fertilization has occurred, and the diploid zygote (initial cell formed during sexual reproduction) has been formed. Following fertilization, the zygote undergoes a series of mitotic cell divisions resulting in a ball of cells called the “blastula”. This undifferentiated embryonic structure then undergoes a phase of major cell migration called “Gastrulation” (early phase in the embryonic development of most animals), producing a gastrula with 3 separate germs layers: the endoderm, mesoderm and ectoderm.

Endoderm: the Endoderm is formed from cells that migrate toward the center of the developing gastrula. They are initially found as flattened cells, but eventually begin to pile on top of each other in a columnar arrangement. The function of the endoderm is to provide an epithelial lining for the body’s 2 major tubes; the digestive and respiratory tube. The endoderm produces lining for the following structures.
Digestive tube (except for mouth, end of rectum).
Accessory digestive organs (pancreas, liver, gallbladder).
Auditory structures, such as the auditory tube and tympanic cavity.
Respiratory structures.
Urinary bladder and urethra.
Mesoderm: The mesoderm is formed during gastrulation from cells that migrate inward and stop between the inner layer (endoderm) and the outer layer (ectoderm). The evolutionary significance of the mesoderm is that it led to the development of Coelom. Coelom is another term for body cavity, and refers to the region between an animal’s outer covering, and the outer covering of the gut cavity. The coelom is the region where the body’s internal organs are found. The mesoderm gives rise to many of the “middle You do not have access to view this node”, but is dynamic in its ability to differentiate in typical directions. Cells that stay in the mesoderm most notably give rise to the following structure.
All muscle, bone and connective You do not have access to view this node
Entire vascular and lymphatic system, including blood
Urogenital organs (kidneys, gonads and reproductive ducts)
The dermis (middle layer of the You do not have access to view this node)
Ectoderm: Since it is the outermost layer found during gastrulation, the ectoderm is said to emerge first. The ectoderm is actually comprised of 3 separate layers: the external ectoderm, the neural crest, and the neural tube. The neural crest and neural tubes are important precursors to the nervous system. Located between the external ectoderm and the neural tube, the neural crest contains tissue that will become neurons and cells of the automatic nervous system. The neural tube is the precursor to the central nervous system, which includes the brain and You do not have access to view this node. The ectoderm gives rise to the following structures.
The entire nervous system
Posterior You do not have access to view this node
Adrenal medulla
Cornea and lens
Epidermis of skin and its derivatives (hair, nails, sweat glands)
Nasal, oral and anal epithelium

1. The ectoderm is the start of a tissue that covers the body surfaces. It emerges first and forms from the outermost of the germ layers.

The ectoderm forms: the central nervous system, the lens of the eye, cranial and sensory, the ganglia and nerves, pigment cells, head connective tissues, the epidermis, hair, and mammary glands.



2. The endoderm consists at first of flattened cells, which subsequently become columnar. It forms the epithelial lining of the whole of the digestive tube except part of the mouth and pharynx and the terminal part of the rectum (which are lined by involutions of the ectoderm). It also forms the lining cells of all the glands which open into the digestive tube, including those of the liver and pancreas; the epithelium of the auditory tube and tympanic cavity; the trachea, bronchi, and air cells of the lungs; the urinary bladder and part of the urethra; and the follicle lining of the thyroid gland and thymus.

The endoderm forms: the stomach, the colon, the liver, the pancreas, the urinary bladder, the lining of the urethra, the epithelial parts of trachea, the lungs, the pharynx, the thyroid, the parathyroid, and the intestines.


3. The mesoderm germ layer forms in the embryos of triploblastic animals. During gastrulation, some of the cells migrating inward contribute to the mesoderm, an additional layer between the endoderm and the ectoderm.

The formation of a mesoderm led to the development of a coelom. Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks.

The mesoderm forms: skeletal muscle, the skeleton, the dermis of skin, connective tissue, the urogenital system, the heart, blood (lymph cells), the kidney, and the spleen.