Embryonic Development

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.

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

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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.

Links: Carnegie stage 13 - serial sections | Carnegie stage 22 - serial sections

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 - 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 - outer cell layer
• Embryoblast - inner cell mass
• Implantation
• Bilaminar embryo

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 Stage Table

Stage	Days (approx)	Size (mm)	Images (not to scale)	Events
1	1 (week 1)	0.1 - 0.15		fertilized oocyte, pronuclei
2	2 - 3	0.1 - 0.2		cell division with reduction in cytoplasmic volume, 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
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		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 - 101	11 - 14		finger rays
18	101 - 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 40 weeks)

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

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

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Cite this page: Hill, M.A. (2024, June 26) Embryology Embryonic Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Embryonic_Development

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G