Stage
| Days
|
Size
|
Events & Characteristic signs
|
1
|
1
(week 1)
|
0.1 - 0.15
|
Fertilized oocyte
Polar bodies
Male and female Pronucleus
Formation of the Zygote
|
2
|
|
|
Separation of the blastomeres
Formation of the blastomeres (totipotent to ca. the 12-cell stage)
Morula stage
Compaction (8 to 16 cells)
The polarization of the blastomeres becomes slowly visible.
|
3
|
|
|
After the blastocyst cavity has formed, one speaks of a blastocyst (16 to 32 cells)
Segmentation (inner and outer cell mass)
Embryonic and abembryonic pole
Free blastocyst
Hatching of the blastocyst through disintegration of the zona pellucida
Epiblast and hypoblast are formed and thereby the didermic embryonic disk.
|
4
|
|
|
Syncytiotrophoblast
Cytotrophoblast
Anchoring of the blastocyst on the endometrium and interaction between the embryo and maternal tissue.
Implantation of the blastocyst in the mucosa of the uterus
|
5
|
7 - 12
(week 2)
|
|
Development of the bilaminar germ disk
Genesis of the amniotic cavity and the primary yolk sac (= primary umbilical vesicle)
Invasion of the trophoblasts into the uterine mucosa
Solid trophoblast
Formation of the trophoblast lacunae and complete penetration of the embryo into the endometrium
Lacunar trophoblast
Definitive amniotic cavity
Primary umbilical vesicle (= primary yolk sac)
|
6
|
|
|
Extraembryonic mesoderm
Primitive streak, Primitive node Primitive groove
Gastrulation
Secundary umbilical vesicle
Allantoic diverticulum
Cloacal membrane
Primordial vessels in the embryonic body stalk
Primordial germ cells
Body stalk
|
7
|
15 - 17
(week 3)
|
|
Gastrulation
Notochordal process
Mesoblast immigration
Chordal process
Neural plate
Genesis of the blood vessel system and formation of blood
|
8
|
|
|
Primitive pit
Notochordal canal
Pear shaped embryo
Axial process
Neurenteric canal
Neural groove forming
|
9
|
|
|
Somitogenesis
Embryo with somites (1-3 somites) 1 - 3 neural folds
Pharyngeal membrane
Neural groove
Caudal eminence
Intraembryonic coelom
Cardiac primordium, septum transversum
Placode and auditory pit
Head fold
|
10
|
22 - 23
(week 4)
|
|
Lateral folding
Closure of the neural groove into a neural tube
The 2 first pharyngeal arches appear
Ophthalmic vesicle
Rupture of the pharyngeal membrane
4 - 12 somites neural fold fuses
|
11
|
|
|
Closure of the rostral neuropore
Nasal placode
Ophthalmic vesicle
Auditory pit
13 – 20 somites
|
12
|
|
|
Closure of the caudal neuropore
Third pharyngeal arch
Buds of the upper extremities
Cervical sinus
21–29 somites
|
13
|
28 - 32
(week 5)
|
|
Buds of the lower extremities
Lens placode
Auditory pit
Fourth pharyngeal arch
More than 30 somites leg buds
|
14
|
|
|
Lens pit
Optic cup
Opthalmic vesicle
Genesis of the endolymphatic canal
Augmented
cephalo-cervical flexure
Cerebellar primordium
Visible ocular primordium
Sixth pharyngeal arch
|
15
|
|
|
Nasal pit
Lens vesicle covered with the ectoderm
Auditory primordium
Genesis of the hand plate
|
16
|
37 - 42
(week 6)
|
|
Nasal pits moved ventrally, auricular hillocks, foot plate
Deep, ventrally lying nasal pit
Pigmentation of the eye
Genesis of the foot plate
Physiologic umbilical hernia
Nasolacrimal groove
|
17
|
|
|
External acoustic meatus
The circumference of the head increases
The trunk lengthens
Fusion of the 6 auricular hillocks
Atrophy of the embryonic tail
Formation of the interdigital zones
Finger rays
|
18
|
44 - 48
(week 7)
|
|
Ossification commences
Toe primordium
Nasal primordium
The elbow joint forms
Gonadal gender can be determined in male embryos
|
19
|
|
|
Straightening of trunk
Lengthening and straightening of the torso
Appearance of the external acoustic meatus
Genesis of the hip and knee joints
Atrophied embryonic tail
Rupture of the cloacal membrane
Maxilla and mandibula
|
20
|
51 - 53
(week 8)
|
|
Upper limbs longer and bent at elbow
Arms in pronation
Separated fingers
Gonadal gender determinable in female embryos
|
21
|
|
|
Subcutaneous vessel network of the head spreads out
Hands and feet come closer and touch each other
Hands and feet turned inward
|
22
|
|
|
Development of the eyelids and the external ear canal
Tragus and antitragus
Nasal septum
Secondary palate
|
23
|
56 - 60
|
27 - 31
|
Rounded head
Body and limbs
Angle between the head and trunk amounts to 30°
The head makes 50% of the embryo's size
The head is rounded, the extremities lengthened and stand out from the body
Chin
Nasal pit
Development of the external genital primordium
|
|
|
|
|