Early Human Development

Question 1

Pre-embryonic period

Answer 1

First two weeks

1) Zygote
2) Morula
3) Blastocyst

Question 2

Zygote

Answer 2

• One cell
• Diploid
• Undergoes rapid mitotic divisions called cleavage divisions

Question 3

Morula

Answer 3

• 72 hours
• Solid ball of 16-32 cells (called blastomeres)
• Same size as ovum/zygote

Question 4

Blastocyst

Answer 4

• Fluid filled cavity develops = blastocoel/ blastocyst cavity
• cells change to become more specialized to form the embryo
• Trophoblast-> divide and differentiate to form Chiron (provides nutrients to embryo)

Question 5

Implantation

Answer 5

• Blastocyst moves from uterine tube and attaches to endometrium of uterine wall
• about 5-7 days

Question 6

After implantation

Answer 6

• Inner cell mass forms a 2-layered embryonic disc composed of:
Epiblast
Hypoblast

Question 7

Epiblast

Answer 7

•Will undergo mitosis to form:

1) Amnion- with a fluid amniotic cavity
2) 3 Germ layers of the embryo (Ectoderm, Mesoderm and Endoderm)

Question 8

Hypoblast

Answer 8

•Will undergo mitosis to form the yolk sac

Question 9

Embryonic Period

Answer 9

•Third to eighth week
• 3 germ layers now present forming the embryo
1) Ectoderm
2) Mesoderm
3) Endoderm
• Everything is almost completely formed Major organs, heart beats, limb buds differentiate and embryonic membranes

Question 10

Ectoderm

Answer 10

•Will form the nervous system and the epidermis of the skin

Question 11

Mesoderm

Answer 11

•Will form: 
muscles
cardiovascular system
dermis
most bone

Question 12

Endoderm

Answer 12

• Will form epithelial linings of digestive, respiratory, urinary and reproductive systems and their glands

Question 13

Embryonic membranes

Answer 13

Chorion
Amino
Yolk sac
Allantois

Question 14

Chorion

Answer 14

-Derived from trophoblast cells
- surrounds all membranes
- chorionic villi= protections from the chorion containing blood vessels
that Will form fetal part of placenta

Question 15

Amnion

Answer 15

Derived from epiblast

• forms amniotic cavity
• fetus in amniotic fluid ( helps absorb shock and temperature regulation

Question 16

Yolk Sac

Answer 16

Derived from hypoblast

• forms part of gut
• produces earliest blood cells and vessels
• source of primordial germ cells

Question 17

Allantois

Answer 17

-Forms the umbilical cord and the urinary bladder

Question 18

Fetal Period

Answer 18

• 9th to 40th week
• growth occurs
• Maturation or organs occur