Back to Biology 1402 pageAnimal Development
Chapter 47
Theories of Animal Development
Preformation - Sperm or embryo contains all its decendents - series of of successively smaller embryos within embryos - "homumculus" - now discredited
Epigenesis - progressive development in form of embryo from relatively formless egg - originally proposed by Aristotle - now widely accepted
Stages in Development
Fertilization - union of male and female gametes
Cleavage - succession of rapid cell divisions without growth during early embryonic development
Gastrulation - formation of 3-layered embryonic stage with primitive gut
Organogenesis - formation of organs from primary germ layers
Two Key Functions
of Fertilization
Combines haploid sets of chromosomes from parents into diploid zygote
Contact of sperm with egg initiates metabolic reactions within egg - "activates" egg and triggers onset of embryonic development
Events Triggered by Fertilization - Acrosomal Reaction
Following contact - acrosomal process of sperm extends through jelly coat of egg - binds to receptors on vitelline layer
Plasma membranes of sperm and egg fuse - sperm nucleus enters cytoplasm of egg
Depolarization of membrane & entry of sodium ions blocks further entry of sperm - fast block to polyspermy
Events Triggered by Fertilization - Cortical Reaction
Fusion triggers signal-transduction pathway - calcium ions released by endoplasmic reticulum into cytoplasm of egg
Cortical granules fuse with plasma membrane
Vitelline membrane separates from plasma membrane - hardens - becomes fertilization membrane - prevents further entry of sperm - slow block to polyspermy
Events Triggered by Fertilization
- Activation of Egg
Rise in calcium levels in egg cytoplasm causes rapid increase in rates of cellular respiration and protein synthesis in egg - "activates" egg
Sperm and egg nuclei merge - create diploid nucleus of zygote
Followed by 3 successive stages - cleavage, gastrulation & organogenesis
Cleavage
Partitions zygote into many smaller cells - blastomeres - creates blastula
Polarity of eggs - animal and vegetal poles - influences plane of division
Meroblastic - incomplete division of yolk-rich egg
Holoblastic - complete division of eggs having little or moderate amount of yolk
Gastrulation
Net effect - invaginations move cells located at or near surface move to interior - creates primitive gut or archenteron - transforms blastula into gastrula
Produces 3 embryonic germ layers - ectoderm, endoderm, & mesoderm - triploblastic condition
Organogenesis
Organs form from 3 embryonic germ layers
Ectoderm - epidermis, nervous system, endocrine glands
Endoderm - lining of digestive and respiratory systems, reproductive system, endocrine glands
Mesoderm - notochord; muscular, circulatory, lymphatic, excretory, reproductive systems; lining of coelom