Morphogenesis Vertebrate eggs Early chordate development Fate of the germ layers Extraembryonic membranes

Vertebrate Eggs Size and clevage pattern depends on amount of yolk present Microlecithal - “little yolk” Mesolecithal - “moderate” yolk Macrolecithal - “large” yolk

Distribution of yolk: Isolecithal - even distribution Telolecithal - yolk is more concentrated in one hemisphere

Disposition of eggs: Oviparity- eggs are shed into the environment Viviparity- eggs are retained internally and living young delivered - Ovoviviparous: young nourished by yolk - Euviviparous: young nourished by maternal tissues (histotrophic or placental)

Early Embryo Development Clevage Refers to division of cells Pattern determined by amount and distribution of yolk (1) Holoblastic: entire egg cleaves (equal or unequal) (2) Meroblastic: only a small disc of cytoplasm cleaves

Blastula Hollow ball of cells (holoblastic) or a “cap” of blastoderm (meroblastic) Each cell is a blastomere Cavity is the blastocoele Massive amounts of yolk result in formation of the blastoderm Placental mammals produce a blastocyst consisting of trophoblast and inner cell mass

Gastrulation The most important event in your life Dynamic process of cellular movements which form the three primordial germ layers and the coelom Morphogenetic movements depend on the amount and distribution of yolk. Involves some type of involution. Regulated by homeobox genes and their proteins - morphogens

Primary Germ Layers Formed during gastrulation Ectoderm: outerlayers of integument and nervous system (neural crest and neurectoderm) Mesoderm: muscle, bone and connective tissue. Dorsal (somite) mesoderm, intermediate and lateral Endoderm: lining of the gut (yolk sac endoderm - primordial germ cells

Extraembryonic Membranes Vitelline: surrounds yolk sac; highly vascular and confluent with embryonic circulatory system Amnion: encloses embryo to protect against dessication (water loss); terrestrial vertebrates Chorion: surrounds amnion, contacts maternal tissue or egg shell Allantois: evagination from cloaca; forms part of mammalian bladder.