Embryonic Development

Embryology - 13 Aug 2026        Expand to Translate
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Introduction

Author Comments

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 the associated developmental 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 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. Human Embryo - Biological definition   Modern Definition The following biological definition comes from the Australian National Health and Medical Research Council (NHMRC) discussion paper (2006). "human embryo means a discrete entity that has arisen from either: (a) the first mitotic division when fertilisation of a human oocyte by a human sperm is complete; or (b) any other process that initiates organised development of a biological entity with a human nuclear genome or altered human nuclear genome that has the potential to develop up to, or beyond, the stage at which the primitive streak appears; and has not yet reached 8 weeks of development since the first mitotic division." This definition was also published later by the same group in 2007. Historical Definition "The distinction between the embryonic and the fetal periods at 8 postovulatory weeks has proved valuable. It is based primarily on the probability that more than 90 percent of the more than 4,500 named structures of the adult body have appeared by that time." Links: Embryonic Development | Fertilization | NHMRC paper PDF Week: 1 2 3 4 5 6 7 8 Carnegie stage: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Kyoto Collection		Carnegie Collection
Human Embryo, Carnegie stages 1-23		Human Embryo, Carnegie stages 10-23

<html5media height="530" width="375">File:Embryo stages 003.mp4</html5media>

Use the 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. Embryo sizes - stage 14 compared to 23  ‎‎Human Embryo Page | Play Embryo Week: Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9 Links: Embryo Virtual Slides | Human Embryo Collections | Carnegie stage 13 - serial sections | Carnegie stage 22 - serial sections

Week 1

Week 1 - Early zygote

• Week 1 Carnegie stage - 1 | 2 | 3 | 4
• oocyte | spermatozoa | fertilization
• zygote
• morula
• blastocyst

Carnegie stages
stage 1	stage 2	stage 3	stage 4

Week 2

Week 2 Implantation

• Week 2 Carnegie stage - 5 | 6
• trophoblast - (trophectoderm) outer cell layer
• Embryoblast - inner cell mass
• implantation
• Bilaminar embryo

Carnegie stages
stage 4	stage 5	stage 6

Week 3

Week 3 - Embryonic disc

• Week 3 - Carnegie stage - 7 | 8 | 9

• Gastrulation

• Notochord

• Trilaminar embryo

• Mesoderm
• Somitogenesis

• Neurogenesis

Carnegie stages
stage 7	stage 8	stage 9

Week 4

Week 4 - The heart begins

• Week 4 - Carnegie stage - 10 | 11 | 12 | 13

• Heart
• Placodes
• Pharyngeal arches

Carnegie stages
stage 10	stage 11	stage 12	stage 13

Week 5

Week 5 embryo

• Week 5 - Carnegie stage - 14 to 15

Carnegie stages
stage 14	stage 15

Week 6

Week 6 - Early face deevelopment

• Week 6 - Carnegie stage - 16 to 17

Carnegie stages
stage 16	stage 17

Week 7

Week 7 - Head and limb development

• Week 7 - Carnegie stage - 18 to 19

Carnegie stages
stage 18	stage 19

Week 8

Week 8 - Last embryonic stage 23

• Week 8 - Carnegie stage - 20 | 21 | 22 | 23
• Last week of embryonic development.

Carnegie stages
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.

Stage	Days (approx)	Size (mm)	Images (not to scale)	Events
1	1 (week 1)	0.1 - 0.15		fertilized oocyte, zygote, pronuclei
2	2 - 3	0.1 - 0.2		morula cell division with reduction in cytoplasmic volume, blastocyst formation of inner and outer cell mass
3	4 - 5	0.1 - 0.2		loss of zona pellucida, free blastocyst
4	5 - 6	0.1 - 0.2		attaching blastocyst
5	7 - 12 (week 2)	0.1 - 0.2		implantation
6	13 - 15	0.2		extraembryonic mesoderm, primitive streak, gastrulation
7	15 - 17 (week 3)	0.4		gastrulation, notochordal process
8	17 - 19	1.0 - 1.5		primitive pit, notochordal canal
9	19 - 21	1.5 - 2.5		Somitogenesis Somite Number 1 - 3 neural folds, cardiac primordium, head fold
10	22 - 23 (week 4)	2 - 3.5		Somite Number 4 - 12 neural fold fuses
11	23 - 26	2.5 - 4.5		Somite Number 13 - 20 rostral neuropore closes
12	26 - 30	3 - 5		Somite Number 21 - 29 caudal neuropore closes
13	28 - 32 (week 5)	4 - 6		Somite Number 30 leg buds, lens placode, pharyngeal arches
Stage 13/14 shown in serial embryo sections series of Embryology Program
14	31 - 35	5 - 7		lens pit, optic cup
15	35 - 38	7 - 9		lens vesicle, nasal pit, hand plate
16	37 - 42 (week 6)	8 - 11		nasal pits moved ventrally, auricular hillocks, foot plate
17	42 - 44	11 - 14		finger rays
18	44 - 48 (week 7)	13 - 17		ossification commences
19	48 - 51	16 - 18		straightening of trunk
20	51 - 53 (week 8)	18 - 22		upper limbs longer and bent at elbow
21	53 - 54	22 - 24		hands and feet turned inward
Stage 22 shown in serial embryo sections series of Embryology Program
22	54 - 56	23 - 28		eyelids, external ears
23	56 - 60	27 - 31		rounded head, body and limbs
Following this stage Fetal Development occurs until birth (approx 37 weeks)

The embryos shown in the table are from the Kyoto and Carnegie collection and other sources.

Glossary Links

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Cite this page: Hill, M.A. (2026, August 13) Embryology Embryonic Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Embryonic_Development

What Links Here?

© Dr Mark Hill 2026, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G