Egg Membranes and Types of Ova

Study Notes

Three types of egg membranes exist:
- Primary egg membrane: Formed inside the ovary by the egg itself. Examples include the zona pellucida in mammals.
- Secondary egg membrane: Formed by ovarian tissue before the egg leaves the ovary. Mammals have a corona radiata around their eggs.
- Tertiary egg membrane: Formed during the egg's passage through the oviduct. Examples include eggs of birds and toads.

Alecithal: Eggs with no yolk, like human eggs.
Microlecithal: Eggs with a small amount of yolk, like amphioxus eggs.
Mesolecithal: Eggs with a moderate amount of yolk, like toad eggs.
Macrolecithal: Eggs with a large amount of yolk, like bird, insect, and reptile eggs.

Isolecithal: Yolk is evenly distributed in the cytoplasm, like amphioxus eggs.
Telolecithal: Yolk is concentrated at one pole (vegetal pole), as in toad and bird eggs.
Centrolecithal: Yolk is centered in the cytoplasm, like insect eggs.

Human Egg: Shows corona radiata, zona pellucida, vitelline membrane, germinal vesicle, germinal spot, and ooplasm.
Amphioxus Egg: Shows animal pole, nucleus, yolk, and vitelline membrane.
Frog Egg: Shows animal and vegetal hemispheres, jelly-like albumen, nucleus, inner and outer vitelline membranes, and yolky vegetal hemisphere.
Bird Egg: Shows yolk, germinal disc, inner thin albumen, chalaziferous albumen, outer thin albumen, thick albumen, cuticle, air cell, inner and outer shell membranes, and shell.

The union of sperm (1n) and ovum (1n) nuclei to form a zygote (2n).
Types:
- External fertilization: Fertilization occurs in water (fish, amphibians). Embryo develops inside a thin eggshell.
- Internal fertilization: Fertilization occurs on land (reptiles, birds, mammals). Embryo develops inside an eggshell (reptiles, birds) or the uterus (mammals).

Successive mitotic divisions of the zygote forming a blastula (ball of cells).
Planes of Cleavage:
- Meridional: Vertical division from animal to vegetal pole, passing through the center (e.g., amphioxus).
- Vertical: Similar to meridional but does not pass through the center (e.g., toad).
- Horizontal/Equatorial: Divides the zygote into unequal halves, not passing through the center (e.g., amphioxus, toad).

Holoblastic (complete): Cleavage furrows pass through the entire egg.
- Equal holoblastic: Cleavage planes pass through the center of the zygote (e.g., amphioxus).
- Unequal holoblastic: Cleavage planes do not pass through the center (e.g., frog).
Meroblastic (incomplete): Cleavage is confined to a disc or part of the egg (e.g., birds).
Superficial: Cleavage is restricted to the outer cytoplasm rim (e.g., insects).

The blastula shows different regions destined to become specific tissues like ectoderm, mesoderm, and endoderm.

Ectoderm: Forms the nervous system, epidermis, hair, nails, lens of the eye
Mesoderm: Forms the skeletal system, muscles, circulatory, excretory and reproductive systems.
Endoderm: Forms the organs like the liver and pancreas.

Early stages of gastrulation: Endoderm cells become plate-like; ectoderm cells experience epiboly (increased mitotic activity); endoderm doesn't show mitotic activity
Endoderm cells of vegetal pole invaginate.
Notochordal and mesodermal cells invaginate.
Blastocoel disappears; gastrocoel forms.

Presumptive neural ectoderm cells form a neural plate along the mid-dorsal line, sinking below the epidermal ectoderm.
Lateral edges of the neural plate meet, forming a neural tube enclosing a neurocoel.
- Notochord forms from invaginated blastopore cells.
- Mesoderm forms from cells at the roof of the archenteron; the mesoderm separates from the endoderm; longitudinal groove appears; groove deepens and opens into the archenteron.