UNSW Embryology

1

1

0.07

1 cell (in oviduct)

2

2

0.08 x 0.06

2 cells (in oviduct)

3

3

0.08 x 0.05

4 cells (in oviduct)

4

3.5

8-12 cells (in oviduct)

5

3.25

0.08 x 0.04

Morula (in uterus)

6

4

(0.08 x 0.03)

Early blastocyst (in uterus)

7

5

(0.12 x 0.05)

Free blastocyst (in uterus)

8

6

(0.28 x 0.07)

Implanting blastocyst, with trophoblastic cone and inner cell mass; outgrowth of endoderm (hypoblast)

9

6.75

Diplotrophoblast; inner cell mass (pendant), covered with endoderm

10

7.25

(0.3 x 0.1)

Near complete implantation; pendant begins differentiation into embryonic and extra-embryonic parts

11

7.75

(0.5 x 0.1)

Completion of implantation; primary amniotic cyst; ectoplacental cone

12

8.5

(1.04 x 0.26)

Connecting ectochorionic and amniotic cavities; rudiments of amniotic folds; primitive streak; start of 3rd layer formation; blastemas of heart and pericardium

13

9

1.0

Presomite neurula; fusion of chorio-amniotic folds, chorio-amniotic stalk; neural plate; embryo bent dorsally; bud of allantoic stalk

14

9.5

1.5

Somites 1-4 (occipital); pendant with 3 cavities: ectochorionic cyst, exocoelom, and amniotic cavity; ectochorionic cyst collapsing; allantoic stalk projects into exocoelom; embryo bent dorsally

15

10

2

Somites 5-12 (cervical); 1st visceral arch; ectochorionic cyst fused with ectoplacenta and with allantoic stalk; regression of peripheral (distal) yolk sac and trophectoderm (diplotrophoblast); Reichert's membrane; gonia in endoderm; embryo bent dorsally

16

10.5

2.4

Somites 13-20 (upper thoracic); 2 visceral arches; disc and yolk sac placentas; appendicular folds; embryo reverses, curves ventrally

17

11

3.3

Somites 21-25 (lower thoracic); yolk stalk closes at level of 15th somite; primary gonia in mesentery; primitive streak disappears; tail bud becomes organized; arm and leg buds recognizable

18

11.5

3.8

Somites 26-28 (upper lumbar); 3 visceral arches; arm buds recognizable

19

11.75

4.2

Somites 29-31 (lower lumbar); visceral arches I-IV; cervical folds; appendicular folds and buds

20

11.875

5

Somites 32-33 (upper sacral)

21

12

5.1

Somites 34-35 (lower sacral); deep cervical sinuses

22

12.125

5.2

Somite 36 (1st caudal); olfactory pits

23

12.25

5.6

Somites 37-38 (caudal); start of umbilical herniation

24

12.375

6

Somites 39-40 (caudal)

25

12.5

6.2

Somites 41-42 (caudal); occipital somites dispersing; 4 visceral arches; deep cervical sinuses; arm buds at somite levels 8-14, about as high as long; leg buds at somite levels 28-31, smaller; body forms a spiral of about 11/2 turns, the left face and trunk applied to yolk sac, the right side turned toward placenta; tail and allantoic stalk rise to the placenta

26

12.75

7

Somites 43-45 (caudal); mandibular, maxillary, and frontonasal processes; cervical sinuses closing; mammary welts; differentiaion of handplates; arm buds vascularized, brachial nerves entering; beginning of umbilical hernia

27

13

8

Somites 46-48 (caudal); prominent facial processes and clefts; nose-snout projecting; cervical sinuses closed; primordia of mammary glands; round handplates and footplates; larger umbilical hernia

28

13.5

8.5

Somites 49-51 (caudal); 1st visceral cleft transforms into external ear duct; precartilaginous condensations in handplates

29

14

9.5

Somites 52-55 (caudal); auricular hillocks on visceral arches I and II

30

14.5

10.5

Somites 56-60 (caudal); body uncoils; mandibular precartilage; nearly round opening of external ear duct; pleuroperitoneal canal has become very narrow

31

15

12

Somites 61-63 (caudal); facial clefts closed; pleuroperitoneal canal closed; complete diaphragm

32

15.5

14.2

Somite 64 (caudal); pinna turns forward; maximal size of umbilical hernia

33

16

15.5

Somite 65 (usually this is last caudal); snout lifts off chest; last stage of metamorphosis

34

17-18

16-20

1st fetal stage: rapid growth of eyelids (eyes entirely covered at end of 18th day); palate complete; pinna covers ear duct; umbilical hernia withdraws

35 ante-natal

19-22

20-40

2nd fetal stage: sealed eyelids; fetal membranes and placentas reach peak of development; tail grows to 10mm;

Birth occurs (22nd day in rat, 19th day in mouse)

35 post-natal

1-16 post-partum

40-100¹

After birth, fetus becomes a breathing and suckling nestling²; during 1st 16 days (22 to 38 days total age), eyelids remain sealed and external ear ducts plugged with periderm

36 post-natal

17+ post-partum

100+¹

Periderm seals of ears and eyelids vanish; active feeding begins within next 3 days and weaning after 1 week (total weaning age, 45-48 days for rats and mice)

Data For Carnegie Stages Comparison Graph (Species/Days)

Species

Stage

Human

Days

20

22

24

28

30

33

36

40

42

44

48

52

54

55

58

Mouse

Days

9

9.5

10

10.5

11

11.5

12

12.5

13

13.5

14

14.5

15

15.5

16

Rat

Days

10.5

11

11.5

12

12.5

13

13.5

14

14.5

15

15.5

16

16.5

17

17.5