Embryonic Development
Introduction
Author Comments |
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Start here by looking at the external appearance of embryos in sequence from 1 to 23.
It is not so important to memorise the dates, as they are only approximate, but more important to understand growth (size changes) and the development (overall sequence of events) during this period. |
This page shows some key events of human development during the embryonic period of the first eight weeks (weeks 1 - 8) following fertilization. This period is also considered the organogenic period, when most organs within the embryo have begun to form.
There are links to more detailed descriptions which can be viewed in a week by week format, by the Carnegie stages or integrated into a Timeline of human development. Online resources available include: individual images of all Carnegie stages, scanning electron micrographs of the earlier stages, cross-sections showing internal structures at mid- and late-embryonic, 3D reconstructions of internal structures, animations of processes, ultrasound scans and information about abnormalites of development. Note that there is variability in the actual timing of specific events and at the end of this period fetal development begins. |
<Flowplayer height="300" width="226" autoplay="true">Embryo stages 002.flv</Flowplayer> |
Quicktime version |
- Carnegie Stages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | About Stages | Timeline
Use the above stage number links to images and information about each specific stage of human development over the first 8 weeks. The links below give a broad overview of developmental events during each week.
Alternatively, look through development week by week.
Embryo Week: Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9
Week 1
- Week 1 Carnegie stage - 1 | 2 | 3 | 4
- Oocyte | Spermatozoa | Fertilization
- Zygote
- Morula
- Blastocyst
Carnegie stages | |||
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stage 1 | stage 2 | stage 3 | stage 4 |
Week 2
- Week 2 Carnegie stage - 5 | 6
- Trophoblast - outer cell layer
- Embryoblast - inner cell mass
- Implantation
- Bilaminar embryo
Carnegie stages | |
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stage 5 | stage 6 |
Week 3
Carnegie stages | ||
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stage 7 | stage 8 | stage 9 |
Week 4
Carnegie stages | |||
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stage 10 | stage 11 | stage 12 | stage 13 |
Week 5
Carnegie stages | |
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stage 14 | stage 15 |
Week 6
Carnegie stages | |
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stage 16 | stage 17 |
Week 7
Carnegie stages | |
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stage 18 | stage 19 |
Week 8
Carnegie stages | |||
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stage 20 | stage 21 | stage 22 | stage 23 |
- Carnegie Stages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | About Stages | Timeline
Carnegie Stage Table
Weeks shown in the table below are embryonic post ovulation age, for clinical Gestational Age (GA) measured from last menstrual period, add 2 weeks.
(not to scale) |
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fertilized oocyte, zygote, pronuclei | ||||
morula cell division with reduction in cytoplasmic volume, blastocyst formation of inner and outer cell mass | ||||
loss of zona pellucida, free blastocyst | ||||
attaching blastocyst | ||||
(week 2) |
implantation | |||
extraembryonic mesoderm, primitive streak, gastrulation | ||||
gastrulation, notochordal process | ||||
primitive pit, notochordal canal | ||||
Somitogenesis Somite Number 1 - 3 neural folds, cardiac primordium, head fold | ||||
Somite Number 4 - 12 neural fold fuses | ||||
Somite Number 13 - 20 rostral neuropore closes | ||||
Somite Number 21 - 29 caudal neuropore closes | ||||
Somite Number 30 leg buds, lens placode, pharyngeal arches | ||||
lens pit, optic cup | ||||
lens vesicle, nasal pit, hand plate | ||||
nasal pits moved ventrally, auricular hillocks, foot plate | ||||
finger rays | ||||
ossification commences | ||||
straightening of trunk | ||||
upper limbs longer and bent at elbow | ||||
hands and feet turned inward | ||||
eyelids, external ears | ||||
rounded head, body and limbs | ||||
The embryos shown in the table are from the Kyoto and Carnegie collection and other sources.
Glossary Links
- Glossary: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols | Term Link
Cite this page: Hill, M.A. (2024, May 21) Embryology Embryonic Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Embryonic_Development
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G