Fertilization = fusion of sperm + ovum to form a diploid zygote.

Site: Usually the ampullary-isthmic region of the uterine tube; timing and exact location vary by species, and canine oocytes mature after ovulation.

Steps:
1. Capacitation — sperm gain fertilizing ability in female tract.
2. Acrosome reaction — sperm releases enzymes to penetrate zona pellucida.
3. Sperm-egg fusion — first sperm fuses with oocyte membrane.
4. Cortical reaction — zona hardens to block polyspermy.
5. Completion of Meiosis II in oocyte.
6. Pronuclear-envelope breakdown and chromosome union on the first mitotic spindle → zygote (2n).

Types of Eggs (by yolk content):
• Alecithal — no yolk (mammals — nutrition from placenta).
• Microlecithal / Oligolecithal — little yolk (mammals technically, sea urchin).
• Mesolecithal — moderate yolk (amphibians).
• Macrolecithal / Megalecithal — huge yolk (birds, reptiles).

By yolk distribution:
• Isolecithal — yolk evenly spread (mammals).
• Telolecithal — yolk concentrated at vegetal pole (birds).
• Centrolecithal — yolk in centre (insects).

FERTILIZATION & EGG CLASSIFICATION

I. FERTILIZATION:

Site: Usually near the ampullary-isthmic region of the uterine tube; timing is species-specific and is delayed in the bitch because primary oocytes mature after ovulation. Sperm reach the site within 4-12 hours of mating (~30 mins in mouse, ~6 hours in cow).

Pre-fertilization changes in sperm:
• Capacitation — biochemical maturation in female tract; removes glycoprotein coat from sperm head; required to make sperm capable of fertilization.
• Hyperactivation — change in flagellar motion; vigorous swimming.

Stages of fertilization:
1. Sperm penetrates corona radiata — outer ring of follicular cells; uses hyaluronidase from sperm acrosome.
2. Acrosome reaction at zona pellucida — sperm membrane fuses with outer acrosome membrane → releases acrosin + other enzymes that drill through the zona.
3. Sperm membrane fuses with oocyte membrane — the first sperm to do so 'wins'.
4. Cortical reaction — Ca²⁺ wave triggers cortical granules to release contents → zona pellucida hardens (zona reaction) → blocks additional sperm = block to polyspermy.
5. Oocyte completes Meiosis II → female pronucleus forms; 2nd polar body extruded.
6. Sperm nucleus decondenses → male pronucleus forms.
7. Pronuclei replicate DNA; their envelopes break down and parental chromosomes align together at first cleavage — diploid zygote (2n) with mixed paternal + maternal chromosomes.

Significance: Restoration of diploid number; mixing of genes; activation of egg metabolism; chromosomal sex determination (in mammals, XY → male and XX → female; in birds, ZZ → male and ZW → female).

II. CLASSIFICATION OF EGGS (Ova):

BY AMOUNT OF YOLK:
• Alecithal (no yolk) — eutherian mammals (placenta nourishes embryo). Strictly 'oligolecithal' = very little yolk.
• Oligolecithal / Microlecithal — little yolk; mammals, lancelet.
• Mesolecithal — moderate yolk; amphibians (frog).
• Polylecithal / Macrolecithal — large yolk; reptiles, birds, monotremes (platypus).

BY DISTRIBUTION OF YOLK:
• Homolecithal / Isolecithal — yolk evenly distributed; alecithal mammals.
• Telolecithal — yolk concentrated at vegetal pole; protoplasm at animal pole. Two subtypes:
– Moderately telolecithal (frog).
– Heavily telolecithal (bird, reptile — yolk fills 99% of egg, embryo develops as a tiny disc on top = blastoderm).
• Centrolecithal — yolk in centre, cytoplasm at periphery; insects.

BY CLEAVAGE PATTERN (consequence of yolk):
• Holoblastic (complete) cleavage — entire egg divides; alecithal/microlecithal eggs (mammals, lancelet).
– Equal — daughter cells of equal size (mammals, lancelet).
– Unequal — daughter cells unequal size (frog).
• Meroblastic (partial) cleavage — only the cytoplasm divides, yolk does not. Heavy-yolk eggs.
– Discoidal — bird, reptile (cleavage limited to disc on top of yolk).
– Superficial — insect (peripheral cytoplasm divides).

MAMMALIAN EGG SUMMARY: Alecithal/microlecithal, isolecithal → holoblastic equal cleavage → forms morula → blastocyst.