Fertilization Fertilization activates the egg Activation of the egg triggers embryonic development

Acrosomal Reaction The acrosomal reaction occurs in echinoderms Ex. sea urchins Receptors on the vitelline egg layer are specific The reaction is the fast block to polyspermy A depolarization of the membrane stops other sperm from penetrating

The acrosomal and cortical reactions during sea urchin fertilization

Timeline for the fertilization of sea urchin eggs

Activation of The Egg Increase in calcium also triggers an increase in metabolic reaction in the egg Artificial activation of egg can occur by injecting calcium ions

A wave of Ca 2+ release during the cortical reaction

Mammalian Fertilization Most mammals show internal fertilization Sperm has to reach zona pellucida by penetrating follicle cells An acrosomal reaction occurs and sperm cell enters egg Zona pellucida hardens which blocks polyspermy Centrosomes originate from sperm cell Chromosomes share a common spindle during first mitotic division

Fertilization in Mammals

Early Embryonic Development 6 days 2 weeks

Figure 47.6 Cleavage in an echinoderm (sea urchin) embryo

Cross Section of a Frog Blastula

Cleavage Fast mitotic divisions without G1 and G2 phases Results in smaller blastomeres Polar planes of division occur with animal and vegetal poles Holoblastic cleavage is complete division of eggs with little yolk ex: frogs Meroblastic = incomplete division with moderate amt of yolk ex: birds

Gastrulation in a Frog Embryo

Body Symmetry Exhibit cephalization

Bilateral Symmetry – body plan in which only a single, imaginary line can divide the body into two equal halves; Ex: vertebrates Radial Symmetry – body plan in which body parts repeat around the center of the body; Ex: sea stars

Organisms without body cavities are considered acoelomates Figure 32.8c Body covering (from ectoderm) Tissue- filled region (from mesoderm) Digestive tract (from endoderm) Acoelomate. Acoelomates such as flatworms lack a body cavity between the digestive tract and outer body wall. (c)

A pseudocoelom –Is a body cavity derived from the blastocoel, rather than from mesoderm Figure 32.8b Pseudocoelom Muscle layer (from mesoderm) Body covering (from ectoderm) Digestive tract (from ectoderm) Pseudocoelomate. Pseudocoelomates such as nematodes have a body cavity only partially lined by tissue derived from mesoderm. (b)

A true body cavity is called a coelom and is derived from mesoderm; these animals are called coelomates Figure 32.8a Coelom Body covering (from ectoderm) Digestive tract (from endoderm) Tissue layer lining coelom and suspending internal organs (from mesoderm) Coelomate. Coelomates such as annelids have a true coelom, a body cavity completely lined by tissue derived from mesoderm. (a)

Figure 32.7 A comparison of early development in protostomes and deuterostomes

Cleavage in a Frog Embryo

Sea urchin development, from single cell to larva

The Establishment of the Body Axes and the First Cleavage Plane in an Amphibian Becomes Dorsal Side Becomes Posterior End Becomes Anterior End

Many different structures are derived from the three embryonic germ layers during organogenesis Figure ECTODERMMESODERMENDODERM Epidermis of skin and its derivatives (including sweat glands, hair follicles) Epithelial lining of mouth and rectum Sense receptors in epidermis Cornea and lens of eye Nervous system Adrenal medulla Tooth enamel Epithelium or pineal and pituitary glands Notochord Skeletal system Muscular system Muscular layer of stomach, intestine, etc. Excretory system Circulatory and lymphatic systems Reproductive system (except germ cells) Dermis of skin Lining of body cavity Adrenal cortex Epithelial lining of digestive tract Epithelial lining of respiratory system Lining of urethra, urinary bladder, and reproductive system Liver Pancreas Thymus Thyroid and parathyroid glands p. 999

Organogenesis Folding, splitting and clustering of cells begins organogenesis Ectoderm rolls into neural tube Mesodermal blocks form somites along axis of notochord: –Gives rise to vertebrae and muscles of backbone Neural plate folds into the neural tube and becomes central nervous system

Chorion - gas exchange and waste storage, lines the egg shell Allantois – gas exchange and waste storage connects embryo to chorion Yolk sac – food storage vitelline vessels embed into the yolk Amnion – protective fluid filled sac

Human Embryo Brain Heart

Organogenesis in a Frog Embryo

Cleavage, gastrulation, and early organogenesis in a chick embryo

Organogenesis in a Chick Embryo About 56 Hours Old Eye Forebrain Heart Blood Vessels Somites Neural Tube

The Development of Extraembryonic Membranes in a Chick

Early development of a human embryo and its extraembryonic membranes

24 Hour Chick Embryo

48 Hour Chick Embryo

72 Hour Chick Embryo