Figure 7.1. Cledoic egg.
Amniotic egg- amnion present
Reptilian egg- possesses shell(cleidoic)
Microlecithal egg- small amount of yolk
Mesolecithal egg- moderate amount of yolk
Macrolecithal egg- large amounts of yolk
Reptiles, birds, sharks
Types of Cleavage
into two equal sized
cells (e.g., in mammals)
to periphery, usually in
embryos with large
amounts of yolk.
Meroblastic discoidal -
Blastodiscs occur in
some fishes, reptiles, birds, Figure 7.2. Blastodisc.
Figure 7.3. Frog development; stages of embryogenesis (book fig. 5-12)
Initial: fertilized egg 2 cell stage; 4 cell stage;
with fertilization 1st cleavage 4 blastomeres
8 cell stage; 32 cell stage; 64 cell stage;
micromeres and morula blastula with
macromeres blastocoel cavity
16 cell stage (not shown)
Cleavage Stages (cont’d.)
Cells push inward to form gastrula
Eventually eliminates blastocoel
Figure 7.6. Gastrulation.
Archenteron- primitive gut
Gives rise to anus in
Gives rise to mouth in
3 primary germ layers Figure 7.7. Cross section of frog gastrula.
Figure 7.8. Early frog gastrula.
Dorsal lip - invagination of cells
Move around yolk cells
Notochord forms from mesodermal cells
Neural plate thickens and
Neurectoderm- gives rise to
Neural crest cells- between
nerve tube and ectoderm
Dorsal root ganglion, branchial
cartilages cells, and pigment cells
Anterior cells- dermis of face,
cephalic armor, sensory capsules
Figure 7.9. Embryonic neurulation.
Figure 7.10. Neural crest cell differentiation. (Table 5.2; page 189).
Dermatome- associated with ectoderm
Dermis in most of body
Sclerotone- associated with nerve tube
Skeletal and vertebral column
Dorsal mesoderm (somite)
splits to form coelom cavity
Somatic mesoderm -
bound to ectoderm
bound to endoderm
Somatopleure - somatic
mesoderm meets ectoderm
Figure 7.12. Mesoderm specialization (book Fig. 5.6).
Dorsal Mesoderm Differentiation
Dorsal mesoderm (somite)- Epimeric mesoderm
Intermediate mesoderm- Mesomeric mesoderm
Lateral plate mesoderm- Hypomeric mesoderm
Primary Germ Layers
Figure 7.14. Formation of the three primary germ layers.
Germ Layers (cont’d.)
Figure 7.15. Organs derived from the germ layers.
Structures formed from
the three germ layers.
in a representative
Figure 7.18. Extraembryonic membranes of cledoic egg.
Extraembryonic Membrane Formation in a Bird
Figure 7.19. Extraembryonic membranes.
Principles of Embryology
Figure 7.20. Embryological development.