1

0.1 – 0.15

Fertilized oocyte

Polar bodies

Male and female pronucleus

Formation of the zygote


2

0.1 – 0.2

Separation of the blastomeres
(totipotent)

Morula stage

Compaction (8 to 16 cells)

The polarization of the blastomeres
is slowly visible.


3

0.1 – 0.2

Formation of the blastocyst (16 to 32 cells) with the blastocyst cavity

Segmentation
(inner and outer cell mass)

Hatching of the blastocyst

Didermic embryonic disk


4

0.1 – 0.2

Syncytiotrophoblast

Cytotrophoblast

Anchoring of the blastocyst on the
endometrium and interaction between the embryo and maternal tissue


5a

0.1

Genesis of the amniotic cavity
and the primary umbilical vesicle

Solid trophoblast


5b
0.1

Lacunar trophoblast

Definitive amniotic cavity

Primary umbilical vesicle


5c
0.15 – 0.2

Extra-embryonic mesoblast

Anlage of the secondary umbilical vesicle


6

0.2

Primitive streak, Primitive node,
Primitive groove

Secondary umbilical vesicle

Allantoic diverticulum

Cloacal membrane


7

0.4

Chordal process

Neural plate

Genesis of the blood vessel system
and formation of blood


8

1.0 – 1.5

Pear shaped embryo

Axial process

Neurenteric canal

Neural folds form neural groove


9

1.5 – 2.5

Formation of somites

Oropharyngeal membrane

Intraembryonic coelom

1-3 somites


10

2 – 3.5

The 2 first pharyngeal arches appear

Rupture of the pharyngeal
membrane

4 - 12 somites


11

2.5 – 4.5

Closure of the rostral neuropore

Nasal placode

Ophthalmic vesicle

13 - 20 somites


12

3 – 5

Closure of the caudal neuropore

Third pharyngeal arch

Buds of the upper extremities

Cervical sinus

21-29 somites


13

4 – 6

Buds of the lower extremities

Lens placode

Fourth pharyngeal arch

More than 30 somites


14

5 – 7

Augmented cephalo-cervical flexure

Cerebellar primordium

Visible ocular primordium

Sixth pharyngeal arch


15

7 – 9

Auditory primordium

Hand plate


16

8 – 11

Pigmentation of the eye

Genesis of the foot plate

Physiologic umbilical hernia


17

11– 14

External acoustic meatus

Formation of the interdigital zones

Atrophy of the embryonic tail


18

13 – 17

The eyelids arise

Toe primordium

Nasal primordium

Gonadal gender can be determined
in male embryos


19

16 – 18

Lengthening and straightening
of the trunk

External acoustic meatus

Rupture of the cloacal membrane


20

18 – 22

Separated fingers

Gonadal gender determinable
in female embryos


21

22 – 24

Subcutaneous vessel network
of the head spreads out

Hands and feet come closer
and touch each other


22

23 – 28

Development of the eyelids
and the external ear canal

Tragus and antitragus

Nasal septum

Secondary palate


23

27 – 31

The head makes 50%
of the embryo's size

Chin

Nasal pit

Development of the external
genital primordium