Early Development Gametes

Sperm

Egg Lecithal = Yolk

Microlecithal egg Small amount of yolk Amphioxus Eutherians

Mesolecithal egg Medium amount of yolk Amphibians

Macrolecithal egg Large amount of yolk Bird and reptiles Most fish

Isolecithal Even yolk distribution In microlecithal eggs

Telolecithal Uneven yolk distribution Macrolecithal and Mesolecithal eggs Vegetal Pole – yolk region Animal Pole – relatively yolk-free, high metabolic activity/embryo

Amniote eggs Amnion Allantoic cavity Amnionic cavity Allantois Albumin Chorion Yolk

Layers around egg Vitelline membrane Jelly Capsule Shell Albumin Corona Radiata Zona Pellucida

Oviparous

Viviparous

Ovoviviparous

Fertilization

Internal Fertilization Apodans Urodeles Amniotes

External Fertilization Fish Frogs

Zygote

Cleavage & Blastula Microlecithal Eggs Cleavage – Mitosis divisions Blastomeres Blastocoel

Cleavage & Blastula Blastocyst in mammals Inner cell mass in mammals Trophoblast cells in mammals

Cleavage & Blastula Mesolecithal eggs

Cleavage & Blastula Macrolecithal Eggs Blastoderm Blastocoel

Gastrulation Germ layers form from which ALL future organs form Notochord forms Bilateral symmetry established

Gastrulation/Microlecithal eggs Involution Blastopore Archenteron

Germ layers Ectoderm –outer layer Mesoderm – middle layer Forms notochord Splits to form coelom Endoderm – inner layer around archenteron

Gastrulation Mesolecithal eggs Epiboly

Ectoderm Nervous System Sensory structures Neural crest cells that become melanocytes, adrenal gland… Epidermis of skin Epithelium of mouth/nose and anus

Endoderm Lungs & Swim bladders Digestive viscera

Mesoderm Chordomesoderm becomes notochord

Mesoderm Dorsal Mesoderm = Epimere Segmented bands called somites Divides into Dermatome Myotome Sclerotome

Mesoderm Lateral plate mesoderm = hypomere Splits into Somatic and Splanchnic layers Coelom between these layers

Hypomere Somatic Mesoderm plus Ectoderm = Somatopleure Splanchnic Mesoderm plus Endoderm = Splanchnopleure

Mesoderm Intermediate mesoderm = Mesomere Kidney tubules and associated ducts

Key Points Which germ layer (ectoderm, endoderm, mesoderm) gives rise to the following structures: Heart Liver Lung Biceps muscle Notochord Brain Kidney Spinal cord Skin

Gastrulation in Macrolecithal eggs Delamination Blastoderm forms upper sheet of cells called Epiblast and Lower sheet of cells called Hypoblast Epiblast becomes Ectoderm Hypoblast becomes Endoderm

Gastrulation in Macrolecithal eggs Mesoderm forms by Primitive Streak Cells stream inward from posterior to anterior Gives rise to notochord

Gastrulation in macrolecithal eggs Some mesoderm is unorganized and migrates, called MESENCHYME

Gastrulation in macrolecithal eggs Body stalk Connection from body to yolk

Gastrulation in mammal Blastoderm Delamination to form hypoblast & epiblast Primitive streak forms mesoderm Notochord Mesenchyme Coelom from splitting of lateral plate mesoderm

Neurulation Dorsal hollow nerve cord Neural crest cells branch off

Organogenesis Beginning of all major organs of the body

Extraembryonic membranes Fish Body stalk Yolk sac

Amphibians

Extraembryonic membranes AMNIOTES Yolk sac (from splanchnopleure)

Amniotes Amnion From somatopleure Amniotic Fluid

Amniotes Chorion From somatopleure For communication with oxygen source Helps form placenta in mammals Against shell in birds

Amniotes Allantois From splanchnopleure Gas exchange in reptiles Waste receptacle in eutherians

Placenta in Eutherians Excellent waste removal & nutrient uptake Mom’s uterus plus baby’s extraembryonic membrane Attaches to baby via umbilical cord Yolk sac functions as placenta in marsupials