1. Early Development
Gametes

2. Sperm

3. Egg
Lecithal = Yolk

4. Microlecithal egg
Small amount of yolk
Amphioxus
Eutherians

5. Mesolecithal egg
Medium amount of yolk
Amphibians

6. Macrolecithal egg
Large amount of yolk
Bird and reptiles
Most fish

7. Isolecithal
Even yolk distribution
In microlecithal eggs

8. Telolecithal Uneven yolk distribution
Macrolecithal and Mesolecithal eggs
Vegetal Pole – yolk region
Animal Pole – relatively yolk-free, high metabolic activity/embryo

9. Amniote eggs Amnion Allantoic cavity Amnionic cavity Allantois Albumin
Chorion
Yolk

10. Layers around egg Vitelline membrane Jelly Capsule Shell Albumin
Corona Radiata
Zona Pellucida

11. Oviparous

12. Viviparous

13. Ovoviviparous

14. Fertilization

15. Internal Fertilization
Apodans
Urodeles
Amniotes

16. External Fertilization
Fish
Frogs

17. Zygote

18. Cleavage & Blastula Microlecithal Eggs
Cleavage – Mitosis divisions
Blastomeres
Blastocoel

21. Cleavage & Blastula Blastocyst in mammals Inner cell mass in mammals
Trophoblast cells in mammals

24. Cleavage & Blastula Mesolecithal eggs

27. Cleavage & Blastula Macrolecithal Eggs
Blastoderm
Blastocoel

30. Gastrulation Germ layers form from which ALL future organs form
Notochord forms
Bilateral symmetry established

31. Gastrulation/Microlecithal eggs
Involution
Blastopore
Archenteron

33. Germ layers Ectoderm –outer layer Mesoderm – middle layer
Forms notochord
Splits to form coelom
Endoderm – inner layer around archenteron

34. Gastrulation Mesolecithal eggs
Epiboly

36. Ectoderm Nervous System Sensory structures
Neural crest cells that become melanocytes, adrenal gland…
Epidermis of skin
Epithelium of mouth/nose and anus

39. Endoderm
Lungs & Swim bladders
Digestive viscera

42. Mesoderm
Chordomesoderm becomes notochord

44. Mesoderm Dorsal Mesoderm = Epimere Segmented bands called somites
Divides into
Dermatome
Myotome
Sclerotome

48. Mesoderm Lateral plate mesoderm = hypomere
Splits into Somatic and Splanchnic layers
Coelom between these layers

50. Hypomere Somatic Mesoderm plus Ectoderm = Somatopleure
Splanchnic Mesoderm plus Endoderm =
Splanchnopleure

52. Mesoderm Intermediate mesoderm = Mesomere
Kidney tubules and associated ducts

55. Key Points
Which germ layer (ectoderm, endoderm, mesoderm) gives rise to the following structures:
Heart
Liver
Lung
Biceps muscle
Notochord
Brain
Kidney
Spinal cord
Skin

56. Gastrulation in Macrolecithal eggs
Delamination
Blastoderm forms upper sheet of cells called Epiblast and
Lower sheet of cells called Hypoblast
Epiblast becomes Ectoderm
Hypoblast becomes Endoderm

59. Gastrulation in Macrolecithal eggs
Mesoderm forms by Primitive Streak
Cells stream inward from posterior to anterior
Gives rise to notochord

64. Gastrulation in macrolecithal eggs
Some mesoderm is unorganized and migrates, called MESENCHYME

65. Gastrulation in macrolecithal eggs
Body stalk
Connection from body to yolk

68. Gastrulation in mammal
Blastoderm
Delamination to form hypoblast & epiblast
Primitive streak forms mesoderm
Notochord
Mesenchyme
Coelom from splitting of lateral plate mesoderm

71. Neurulation
Dorsal hollow nerve cord
Neural crest cells branch off

74. Organogenesis
Beginning of all major organs of the body

75. Extraembryonic membranes
Fish
Body stalk
Yolk sac

76. Amphibians

77. Extraembryonic membranes
AMNIOTES
Yolk sac (from splanchnopleure)

80. Amniotes
Amnion
From somatopleure
Amniotic Fluid

83. Amniotes Chorion From somatopleure
For communication with oxygen source
Helps form placenta in mammals
Against shell in birds

85. Amniotes Allantois From splanchnopleure Gas exchange in reptiles
Waste receptacle in eutherians

88. Placenta in Eutherians
Excellent waste removal & nutrient uptake
Mom’s uterus plus baby’s extraembryonic membrane
Attaches to baby via umbilical cord
Yolk sac functions as placenta in marsupials