| 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.
Clicking the Carnegie stage numbers opens a page dedicated to describing that single stage and events.
| 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.
Carnegie Stages1 | 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.
- Links: Human Embryo Collections | Carnegie stage 13 - serial sections | Carnegie stage 22 - serial sections
Alternatively, look through development week by week.
|stage 1||stage 2||stage 3||stage 4|
- Week 2 Carnegie stage - 5 | 6
- Trophoblast - outer cell layer
- Embryoblast - inner cell mass
- Bilaminar embryo
|stage 5||stage 6|
|stage 7||stage 8||stage 9|
|stage 10||stage 11||stage 12||stage 13|
|stage 14||stage 15|
|stage 16||stage 17|
|stage 18||stage 19|
|stage 20||stage 21||stage 22||stage 23|
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)
|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|
|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|
|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.
- 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
Cite this page:
Hill, M.A. (2013) Embryonic Development. Retrieved June 19, 2013, from http://php.med.unsw.edu.au/embryology/index.php?title=Embryonic_Development
- Dr Mark Hill 2013, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G