Early Development

Gametes

Sperm

Egg

Lecithal = Yolk

Microlecithal egg

Small amount of yolk

Amphioxus

Eutherians

Mesolecithal egg

Medium amount of yolkAmphibians

Macrolecithal egg

Large amount of yolkBird and reptilesMost fish

Isolecithal

Even yolk distributionIn microlecithal eggs

Telolecithal

Uneven yolk distributionMacrolecithal and Mesolecithal eggsVegetal Pole – yolk regionAnimal Pole – relatively yolk-free, high

metabolic activity/embryo

AmnionAmnion

Amnioniccavity

Amnioniccavity

AllantoisAllantois

AllantoiccavityAllantoiccavity

ChorionChorionYolkYolk

Amniote eggsAmniote eggs

AlbuminAlbumin

Layers around egg

Vitelline membraneJellyCapsuleShellAlbuminCorona RadiataZona Pellucida

Oviparous

Viviparous

Ovoviviparous

Fertilization

Internal Fertilization

ApodansUrodelesAmniotes

External Fertilization

FishFrogs

Zygote

Cleavage & BlastulaMicrolecithal Eggs

Cleavage – Mitosis divisions

Blastomeres

Blastocoel

Cleavage & Blastula

Blastocyst in mammalsInner cell mass in mammalsTrophoblast cells in mammals

Cleavage & BlastulaMesolecithal eggs

Cleavage & BlastulaMacrolecithal Eggs

BlastodermBlastocoel

Gastrulation

Germ layers form from which ALL future organs form

Notochord formsBilateral symmetry established

Gastrulation/Microlecithal eggs

InvolutionBlastoporeArchenteron

Germ layers

Ectoderm –outer layerMesoderm – middle layer

– Forms notochord– Splits to form coelom

Endoderm – inner layer around archenteron

GastrulationMesolecithal eggs

Epiboly

Ectoderm

Nervous SystemSensory structuresNeural crest cells that become melanocytes,

adrenal gland…Epidermis of skinEpithelium of mouth/nose and anus

Endoderm

Lungs & Swim bladdersDigestive viscera

Mesoderm

Chordomesoderm becomes notochord

Mesoderm

Dorsal Mesoderm = EpimereSegmented bands called somitesDivides into

– Dermatome– Myotome– Sclerotome

Mesoderm

Lateral plate mesoderm = hypomereSplits into Somatic and Splanchnic layersCoelom between these layers

Hypomere

Somatic Mesoderm plus Ectoderm =SomatopleureSplanchnic Mesoderm plus Endoderm =Splanchnopleure

Mesoderm

Intermediate mesoderm = MesomereKidney 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

DelaminationBlastoderm forms upper sheet of cells

called Epiblast andLower sheet of cells called HypoblastEpiblast becomes EctodermHypoblast becomes Endoderm

Gastrulation in Macrolecithal eggs

Mesoderm forms by Primitive StreakCells stream inward from posterior to

anteriorGives rise to notochord

Gastrulation in macrolecithal eggs

Some mesoderm is unorganized and migrates, called MESENCHYME

Gastrulation in macrolecithal eggs

Body stalkConnection from body to yolk

Gastrulation in mammal

BlastodermDelamination to form hypoblast & epiblastPrimitive streak forms mesoderm

– Notochord– Mesenchyme– Coelom from splitting of lateral plate

mesoderm

Neurulation

Dorsal hollow nerve cordNeural 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

AmnionFrom somatopleureAmniotic Fluid

Amniotes

ChorionFrom somatopleureFor communication with oxygen sourceHelps form placenta in mammalsAgainst shell in birds

Amniotes

AllantoisFrom splanchnopleureGas exchange in reptilesWaste receptacle in eutherians

Placenta in Eutherians

Excellent waste removal & nutrient uptakeMom’s uterus plus baby’s extraembryonic

membraneAttaches to baby via umbilical cordYolk sac functions as placenta in marsupials