1. Lecture #20 Date ________ u Chapter 47 ~ Animal Development

2. u Fertilization – sperm and egg combine u Cleavage – cell divisions u Gastrulation – differentiation u Organogenesis – development of organs

3. Embryonic development/fertilization u Preformation~ until 18th century; miniature infant in sperm or egg u At fertilization/conception: u Acrosomal reaction~ hydrolytic enzyme action on egg jelly coat…. –Fast block to polyspermy~ membrane depolarization prevents multiple fertilizations…. u Cortical reaction~ release of calcium causes hardening of egg outer layer and creates a... –Slow block to polyspermy and... u Egg activation~ increases metabolic activity; protein synthesis

4. The Fertilized Egg & Cleavage u Blastomeres~ resultant cells of cleavage/mitosis u Morula~ solid ball of cells u Blastocoel ~fluid-filled cavity in morula u Blastula ~hollow ball stage of development

5. Cleavage Blastula Blastocoel

6. Setting up the body axes u Yolk ~ nutrients stored in the egg u Vegetal pole ~ side of egg with high yolk concentration u Animal pole ~ side of egg with low yolk concentration u Anterior/posterior sides already determined in egg before fertilization u Dorsal/ventral determined after sperm enters egg –Animal hemisphere on sperm entry side rotates down (cortical rotation) –Exposes the gray crescent which becomes the dorsal side

8. Gastrulation u Gastrula ~ 2 layered, cup-shaped embryonic stage u 3 Embryonic germ layers: u Ectoderm~ outer layer; epidermis; nervous system, etc. u Endoderm ~ inner layer; digestive tract and associated organs; respiratory, etc. u Mesoderm ~skeletal; muscular; excretory, etc. u Invagination ~ gastrula buckling process to create the... u Archenteron ~ primitive gut u Blastopore ~ open end of archenteron

10. Organogenesis: organ formation u Blastodisc ~ cap of cells on top of yolk u Primitive streak ~ invagination of blastodisc u Neural tube ~ beginning of spinal cord u Somites~ vertebrae and skeletal muscles u Neural crest ~ bones and muscles of skull

11. Amniote embryos u Embryo develops the same as the frog but with other membranes u Extraembryonic membranes : yolk sac (support; provides nourishment circulatory function) amnion (fluid-filled sac; protection/cushion) chorion (gas exchange) –allantois (nitrogenous waste)

12. Control of Development u Homeotic genes (homeobox)