00:00 / 00:00
Human development days 1-4
Human development days 4-7
Human development week 2
Human development week 3
Development of the digestive system and body cavities
Development of the fetal membranes
Development of the placenta
Development of the umbilical cord
Development of twins
Hedgehog signaling pathway
0 / 13 complete
0 / 1 complete
During the third week of development, the embryo is a flat, three-layered disc, and each layer contains germ cells that give rise to the organs and tissues of the body.
The ventral, or bottom, germ layer is called the endoderm, and it forms the lining of the gut tube and the respiratory system.
The middle germ layer is called the mesoderm, and it forms connective tissues like muscles and bones.
And the dorsal, or top, germ layer is called the ectoderm, and it gives rise to the sensory organs like the skin, the eyes, and the central nervous system.
The mesoderm differentiates into the paraxial mesoderm, intermediate mesoderm, and lateral plate mesoderm, and, at around day 19, a space forms in the lateral plate mesoderm.
This space, called the intraembryonic coelom, or cavity, separates the lateral plate mesoderm into dorsal and ventral layers.
The dorsal layer is the parietal mesoderm layer, and its cells adhere to cells from the overlying ectoderm and wrap around the amnion.
The ventral layer is called the visceral mesoderm layer, and its cells adhere to cells from the underlying endoderm, which covers the yolk sac.
The parietal mesoderm gives rise to the serous membranes that line the different cavities in the body, and the visceral mesoderm becomes the serous membrane that covers the lungs, heart, and abdominal organs.
These membranes are really important because they prevent the organs from getting injured as they rub up against each other or against the body wall.
The primitive digestive system starts developing at around week 3 in the embryo. The primitive gut starts to form when the yolk sac becomes incorporated into the embryo. It then forms a blind-ended tube, which becomes the foregut on the cephalic end, and the hindgut on the caudal end. The middle part of the midgut also forms but remains temporarily attached to the yolk sac via the vitelline duct (yolk stalk). The accessory organs of the digestive system form as outpouchings alongside the primitive gut tube.
Latest on COVID-19
Nurse Practitioner (NP)
Physician Assistant (PA)
Create custom content
Raise the Line Podcast
Copyright © 2024 Elsevier, its licensors, and contributors. All rights are reserved, including those for text and data mining, AI training, and similar technologies.
Cookies are used by this site.
Terms and Conditions
USMLE® is a joint program of the Federation of State Medical Boards (FSMB) and the National Board of Medical Examiners (NBME). COMLEX-USA® is a registered trademark of The National Board of Osteopathic Medical Examiners, Inc. NCLEX-RN® is a registered trademark of the National Council of State Boards of Nursing, Inc. Test names and other trademarks are the property of the respective trademark holders. None of the trademark holders are endorsed by nor affiliated with Osmosis or this website.