1. Early Development Review
Zygote = 1 celled stage
Cleavage=early cell division (no cell growth)
Morula = solid multicelled embryo
Blastula = “hollow” multicelled embryo
Blastocoel = space/cavity in blastula
Gastrulation - formation of germ layers
Gastrula - embryo gastrulating
Germ Layers - ecto-, endo-, & mesoderm

2. Early Development Review
Neurulation - formation of nervous system from ectoderm
Neurula - embryo neurulating

3. Cleavage and Blastula

4. Zygote & Cleavage Yolk (lecithin) = food storage
Animal pole = less yolky end of zygote
Vegetal pole = yolky end of zygote
Holoblastic cleavage = zygote divides completely (plesiomorphic for vertebrates)
Meroblastic cleavage = yolky region does not divide (apomorphic, evolved at least 3 times in vertebrates)

5. Cleavage

6. Blastula
Cavity is flat and “above” yolk in species with meroblastic cleavage.
Blastodisc = small cellular area “on” large yolk
In most therian mammals a blastocyst forms.
Blastocyst = a blastula containing a distinct inner cells mass (stem cells)

7. Craniate Phylogeny Myxini (hagfishes) Chondrichthys Actinopterygii
coelacanths
amphibians
lungfishes
lampreys
Mammalia
Reptilia
meroblastic
cleavage
mero. cleavage
mero-
blastic
cleavage

8. Class Actinopterygii sturgeons & paddlefish Amia calva bowfin TELEOSTS
reedfishes
TELEOSTS
gars
meroblastic
cleavage
single, dorsal
lung

9. Amniota lizards & snakes crocodilians montremes marsupials eutheria
turtles
lizards & snakes
crocodilians
birds
*blastocyst
(with inner
cell mass) *
holoblastic
cleavage
meroblastic
cleavage

10. Gastrulation Hollow blastula “pushes” in.
Cells invaginate (pouch in), involute (fold in), or ingress (move in as individual cells) to form a multi-layered embryo.
Forms germ layers.
Method of gastrulation varies among vertebrates.
Amount of yolk affects gastrulation.

11. Lancelet Gastrulation

12. Frog Gastrulation
Similar in lampreys, non-teleost actinopterygiians, lungfishes, & other amphibians

13. Amniote Gastrulation
Primitive Streak = elongate region of ingressing cells in an amniote gastrula
Primitive streak gastrulation modified gastrulation for a large volume of yolk.
Small disk (blastodisc) of cells on top of a large yolk.
Therian Mammals = little yolk but still retain primitive streak gastrulation.

14. Chicken Blastula
animal
pole

15. Primitive Streak Gastrulation

16. Chicken Gastrulation
Posterior
Primitive
Streak

17. Chicken Gastrulation
Node
Posterior
Primitive
Streak

18. Chicken Gastrulation
Node
Posterior
Primitive
Streak

19. Chicken Gastrulation
Node
Posterior
Primitive
Streak

20. Fate Maps

21. Craniate Phylogeny Myxini (hagfishes) Chondrichthys Actinopterygii
coelacanths
amphibians
lungfishes
lampreys
Mammalia
Reptilia
primitive
streak
gastrulation

22. Neurulation
Mesodermal notochord signals overlying ectodermal epithelium to form hollow neural tube.
Neural tube  central nervous system.
Primary neurulation = ectodermal epithelium infolds anteriorly.
Secondary neurulation = occurs in the post-anal tail of (all) vertebrates; mesenchymal mesoderm cells form a rod-shaped mass that cavitates.

23. Neurulation

24. Primary Neurulation

25. Primitive Streak Gastrulation

26. Amniote Gastrulation
Node
Posterior
Primitive
Streak

27. Amniote Neurulation
Node
Neural
Fold
Posterior
Primitive
Streak

28. Amniote Neurulation Neural Node Tube (brain) Neural Fold Primitive
Streak
Posterior

29. Vertebrate Ectoderm Generalized Ectoderm = covers embryo.
Neurectoderm = cells that fold in to form neural tube (brain, spinal cord, retina, some nerves).
Neural Crest = migratory ectodermal tissue; forms in neurulation (pigment cells, “face” skeleton, ganglionic nerves, meninges, & chromaffin cells of adrenal glands).
Ectodermal Placodes = ectodermal epithelium thickens to form most of the sense organs (and some head nerve ganglia).

30. Neural Crest Migration

31. Craniate Phylogeny Myxini (hagfishes) Chondrichthys Actinopterygii
coelacanths
amphibians
lungfishes
lampreys
Mammalia
Reptilia
secondary neurulation posteriorly
neural crest; ectodermal placodes

32. Vertebrate Mesoderm Chordamesoderm = notochord.
Somites (Paraxial) = pouched mesoderm segments formed lateral to notochord.
dermatome, sclerotome, & myotome
Intermediate Mesoderm = lateral & posterior pouched mesodermal segments.
Lateral Plate Mesoderm = ventral unsegmented mesoderm; around coelom.
splanchnic – deep; along endoderm
somatic – superficial; along body wall

33. Vertebrate Mesoderm Somitomeres Somites Lateral Plate Mesoderm
Intermediate
Mesoderm

34. Cephalochordate Mesoderm Formation During Neurulation

35. Vertebrate Mesoderm

36. Somitomeres & Somites
Somitomeres = small, less defined head somites.

37. Vertebrate Neurula (section)
somite
inter-
mediate
meso-
derm
lateral
plate mesoderm
notochord
coelom
gut

38. Vertebrate Mesoderm

39. Vertebrate Neurula (section)
somite
inter-
mediate
meso-
derm
lateral
plate mesoderm
notochord
coelom
gut

40. Vertebrate Embryo (section)
neural
crest
cells
dermatome
sclerotome
myotome
inter-
mediate
meso-
derm
gut
splanchnic
mesoderm
somatic
coelom
lateral
plate mesoderm

41. Vertebrate Embryo (section)
neural
crest
cells
dermatome
sclerotome
myotome
gut
epidermal
ectoderm
coelom

42. Vertebrate Embryo (section)
pigment cell
precursors
(neural crest)
ganglionic
neuron
dermatome
sclerotome
myotome
developing
skin
coelom
gut
dermis
epidermis
neural crest
somatic
(lateral plate) mesoderm

43. Vertebrate Embryo (section)
dermatome
sclerotome
myotome
horizontal
septum
developing
skin
coelom
gut
dermis
epidermis
neural crest
somatic
(lateral plate) mesoderm

44. Vertebrate Embryo (section)
dermatome
vertebra
myotome
horizontal
septum
developing
skin
coelom
gut
dermis
epidermis
neural crest
somatic
(lateral plate) mesoderm

45. Vertebrate Embryo (section)
dermatome
vertebra
myotome
horizontal
septum
developing
skin
coelom
gut
dermis
epidermis
neural crest
somatic
(lateral plate) mesoderm

46. Salmon Section Diagram

47. Lamprey Section

48. Human Thorax Section

49. Mesentaries & “Membranes”
Septum = a “wall”-like division
Membrane = relatively thin sheet of tissue
Mesentary = a relatively thin two-layered sheet of mesothelium with connective tissue between within the coelomic cavity. (General usage)
Mesothelium = simple squamous coelomic epithelium derived from lateral plate mesoderm.

50. Visceral Mesenteries
Dorsal Mesentery – (splancnic mesoderm) supports digestive tract (and blood vessel pathway) from dorsal body wall. [greater omentum/mesogaster, mesentery (proper), mesocolon]
Ventral Mesentery – (splancnic mesoderm) supports digestive tract from ventral body wall. – developmentally lost along most of body except in region of liver [lesser omentum (hepatogastric & hepatoduodenal “ligaments”) and coronal & falciform “ligaments”]

51. Vertebrate Neurula (section)
somite
inter-
mediate
meso-
derm
lateral
plate mesoderm
notochord
gut

52. Vertebrate Embryo (section)
gut
splanchnic
mesoderm
somatic
lateral
plate mesoderm

53. Vertebrate Embryo (section)
gut
splanchnic
mesoderm
somatic
lateral
plate mesoderm

54. Vertebrate Embryo (section)
coelom
splanchnic
lat. pl. mesoderm
somatic
lateral
plate
mesoderm
dorsal &
ventral
mesenteries
(splanchnic
lat. pl. mesoderm)
gut

55. Vertebrate Embryo (section)
coelom
splanchnic
lat. pl. mesoderm
somatic
lateral
plate
mesoderm
dorsal &
ventral
mesenteries
(splanchnic
lat. pl. mesoderm)
gut

56. Vertebrate Embryo (section)
coelom
somatic
lateral
plate
mesoderm
dorsal
mesentery
(splanchnic
lat. pl. mesoderm)
gut
splanchnic
lat. pl. mesoderm

57. Visceral Mesenteries

58. Extraembryonic Membranes
Extraembryonic membranes = membranes that do NOT contribute to the adult body. Shed at birth/hatching.
Chorion in non-amniotes = acellular layer covering developing embryo.
Yolk sac in non-amniotes = membrane continuous with midgut, surrounds yolk mass; composed of endoderm, mesoderm, and ectoderm.

59. Non-Amniote Membranes
chorion
Yolk
Yolk
Sac
endoderm
splanchnic
mesoderm

60. Amniotes 4 extraembryonic membranes.
Yolk sac = endoderm and splanchnic mesoderm; contains yolk
Allantois = endoderm and splanchnic mesoderm; extension of posterior gut
Chorion = ectoderm and somatic mesoderm; surrounds “everything”
Amnion = ectoderm and somatic mesoderm; surrounds embryo

61. Reptile Development Yolk embryo coelom somatic mesoderm endoderm
ecto-
derm
somatic
mesoderm
splanchnic
embryo
Yolk

62. Reptile Development
allantois
somatic
mesoderm
splanchnic
coelom

63. Reptile Development O2 waste CO2 nutrients amnion yolk sac chorion
allantois
amnion
yolk sac
chorion O2
CO2
waste
nutrients

64. Human Development Humans exhibit typical eutherian mammal development.
Holoblastic cleavage leads to blastocyst
Implantation of blastocyst in uterus using a special tissue called trophoblast
Early formation of amnion.
Primitive streak gastrulation
Primary neurulation with secondary neurulation in post-anal tail

65. Human Cleavage

66. Human Early Development

67. Human Blastocyst/Implantation

68. Human Blastocyst/Implantation

69. Human Gastrulation

70. Human Neurulation

71. Human Development

72. Human Extraembryonic Memb.=

73. Human Extraembryonic Memb.

74. Human Extraembryonic Memb.

75. Development
Review

76. Mesoderm Differentiation
Chordamesoderm – notochord
Lateral Plate Mesoderm
somatic mesoderm
splanchnic mesoderm
Intermediate Mesoderm
Somites & Somitomeres (Paraxial)
dermatome
sclerotome
myotome

77. Ectoderm Differentiation
Generalized Ectoderm
Neurectoderm (neural tube)
Neural Crest
Epidermal Placodes
Endoderm