4Craniate Eggs Types of eggs: Microlecithal – iso-lecithal distribution of yolk… found in placental mammals and amphioxusMesolecithal – telolecithal distribution of yolk concentrated at the vegetal pole… found in lampreys, bony fish, amphibiansMacrolecithal – telolecithal eggs… found in marine lampreys, cartilaginous fish, reptiles, monotremes
5Craniate Eggs Oviparity & Viviparity: Animals that “lay” their eggs are considered oviparous.Animals that give birth are considered viviparous.If the embryo could develop without maternal tissue then ovoviviparous, while dependent strategies are euviviparousNo viviparous turtles, crocodiles, or birds
6Craniate Eggs Viviparity: Histotrophic vs. placental nourishment Life in cold blood caecilian clip
7Craniate Eggs Fertilization: Internal and external modes exist In which type of “–parity” are eggs by necessity internally fertilized?Usually external fertilization requires millions and millions of spermUrodeles (salamanders) may use spermatophores and spermatheca
8Early Development Cleavage and the blastula: As fertilized egg cells divide this is called cleavage.What happens to cell size initially?Hollow sphere is called the blast-ula and it contains a hollow space… the blastocoel.Excessive yolk impedes cell division… such that a blastoderm develops on an otherwise undivided yolk.What taxonomic group(s) would utilize this mode?
9Early DevelopmentLet’s review what we can infer since we know this is “Chordate” development…What type of cleavage pattern?What does the 1st opening become?How many germ layers do we have?
10Early DevelopmentGastrulation is the process in which 3 primitive germ layers are formedAmphioxous eggs lack yolk and provide a clear pictureInvolution of blastulaResultant opening is the blastopore
11Early DevelopmentMesoderm forms from the endoderm and then creates pouches (coelomic or mesodermal)Lateral/superficial layer becomes somatic mesodermInner/deep layer becomes splanchnic mesodermWhat becomes of the ectoderm?How about the endoderm?
12Early Development Gastrulation: Frog – because the yolk cells are slow… involution is delayed.Consequently, slightly different process accomplishes same ends.Lateral mesoderm moves in from sides and ventral area of blastopore heading for the head.
13Early Development Gastrulation: Chick – Special consideration required when we have a marolecithal egg.Blastoderm (epiblast and hypoblast)Hypoblast continues around the yolk to become part of the yolk sacEpiblast has cells migrating inward that become endodermCells destined to become the mesoderm migrate in between the other 2 “derms”
14Early DevelopmentWhat regulates differentiation of these cells into various germs, derms, tissues and organs?Organizer areaHomeotic genes (sonic hedgehog gene)Proteins involved in signaling called morphogensDefective homeotic genes can have severe developmental consequences… ex. spina bifida
15Early DevelopmentWhat is the fate of the different “derms”?
16Germ Layers Placental mammals have varying approaches to gastrulation However, generally a neural plate formsNeural folds develop resulting in a neural grooveNeural folds unite forming a neural tubeProcess called Neurulation
17Extraembryonic Membranes Extraembryonic membranes include:Yolk sac, amnion, chorion and allantois
18Extraembryonic Membranes What does a yolk sac accomplish? How?Yolk sac is a highly vascular membrane that surrounds the yolk.Empties into the midgutCan secrete enzymes to digest yolkCan serve as respiratory organ in viviparous amphibians/fishCan absorb nutrients from mother… functions as a simple yolk sac placenta or a “pseudoplacenta”
19Extraembryonic Membranes Reptiles and mammals develop inside 2 sacs… Amnion and Chorion:Amnion surrounds the embryoChorion surrounds the amnion and the yolk sacImportant feature that allows eggs to be laid on land (with less dependence on water)Amniotic fluid surrounds the embryo and is contained by the amnionWhere does this water come from?
20Extraembryonic Membranes Allantois is an evagination of the cloacaCommunicates with the inner surface of the chorion forming the chorioallantoic membraneReptiles and monotremes aids in transferring gases (respiration)In most mammals serves as a membrane of the placenta… transferring nutrients and wastes.Base of this sac becomes the urinary bladder
21Extraembryonic Membranes Placenta generally is any place embryonic and maternal tissues come together for exchangeMore specifically/restricted definition… organ containing highly vascular region of extraembryonic membrane in communication with highly vascular region of maternal tissue
22Extraembryonic Membranes Placentas can have a simple communication with maternal tissue… nondeciduous placentaOrThey can have chorionic villi that invade endometrium and pull part of this lining off with birth… deciduous placentaVilli can be positioned variously on the chorion