The document describes the structure and early development of a fertilized egg. It shows that a fertilized egg contains a zygote surrounded by membranes and air sacs within the egg shell. The zygote undergoes cleavage to form a blastoderm with multiple blastomeres. Gastrulation then occurs as the blastoderm elongates and forms three germinal layers - the epiblast, hypoblast and mesoderm. This establishes the basis for further organ development in the early embryo.

1. Fertilized Egg “Laid” = Zygote
shell
membrane
Chalaza
Air sac
Egg shell
Zygote

2. Structure of Fertilized Egg
Zygote
Air sac
Chataza
shell Egg shell
membrane

3. Cleavage in Chick Embryo
Cleavage 1 Cleavage 2
Zygote Two Blastomeres four Blastomeres
Cleavage 3,4,5
Area Opaca Sixteen
Blastomeres
Area Pellucida
Blastoderm

4. Gastrulation in Chick
Yolk
Blastoderm elogated
Blastodisc or
Blastoderm
blastocoel epiblast
hypoblast

5. Gastrulation in Frog

6. From Gastrula to Neurula

7. Three Germinal Layers in Chick Embryo
Yolk sac
Yolk