Early Development in Animals

1. Fertilization → Zygote

  • Fertilization (conception)

    • Fusion of sperm (n) + ovum (n) → zygote (2n)

  • Zygote = single cell → start of embryonic development

2. Cleavage (Rapid Cell Division)

  • Cleavage

    • Rapid mitotic divisions

    • No increase in overall size

    • Produces smaller cells: blastomeres

Stages

  • 2-cell → 4-cell → 8-cell → 16-cell

  • Morula

    • 16–32 cells

    • Solid, compact mass (mulberry-like)

3. Blastulation

  • Morula develops a cavity → blastocoel

  • Blastula

    • Hollow structure of cells

  • In mammals → blastocyst

    • Inner cell mass → embryo

    • Trophoblast → placenta

4. Implantation

  • Occurs 7–10 days after fertilization

  • Blastocyst embeds in endometrium (uterine lining)

5. Pregnancy Hormone (hCG)

  • Secreted by trophoblast

  • Function:

    • Maintains corpus luteum → progesterone production

  • Detected in:

    • Blood + urine → pregnancy tests

6. Gastrulation (Critical Phase)

  • Occurs ~2–3 weeks after conception

  • Forms three germ layers

Germ Layers and Derivatives

  • Ectoderm

    • Skin

    • Nervous system

  • Mesoderm

    • Muscles

    • Bones

    • Circulatory system

  • Endoderm

    • Digestive tract lining

    • Respiratory system

  • Embryo at this stage = gastrula

7. Neurulation

  • Formation of nervous system

Process

  • Mesoderm → notochord

  • Notochord signals ectoderm → neural plate

  • Neural plate folds → neural tube

Outcome

  • Neural tube → brain + spinal cord

8. Organogenesis

  • Formation of organs from germ layers

  • Begins after gastrulation and neurulation

  • Example:

    • Heart begins beating ~9 weeks

9. Embryo → Fetus

  • Embryo

    • Early developmental stage (first ~8 weeks)

  • Fetus

    • After ~2 months

    • Organs begin functioning

10. Gestation

  • Duration (humans): ~9 months

  • Includes all stages:

    • Cleavage → blastulation → gastrulation → neurulation → organogenesis

11. Developmental Summary (Timeline)

  1. Fertilization → zygote

  2. Cleavage → morula

  3. Blastulation → blastocyst

  4. Implantation

  5. Gastrulation → 3 germ layers

  6. Neurulation → nervous system

  7. Organogenesis → organs

  8. Fetus → growth until birth

12. Variation Across Animals

Mammals

  • Internal development (uterus)

  • Placenta present

Oviparous animals (birds, fish)

  • Development in eggs outside body

Key Differences

  • Yolk distribution

    • Affects cleavage pattern

    • Affects blastula structure

13. High-Yield Summary

  • Cleavage = rapid mitosis, no growth

  • Morula = solid ball

  • Blastocyst = hollow + implantation-ready

  • Gastrulation = 3 germ layers (foundation of body plan)

  • Neurulation = nervous system formation

  • hCG = first detectable pregnancy signal

  • Organogenesis = structure formation