1. Early Craniate Morphogenesis
Why study this material?
Phylogeny
Morphology
Ontogeny

2. What is Life History?
Distinct periods or times? How should we refer to them?
generate list(s) on board

4. Craniate Eggs Types of eggs:
Microlecithal – iso-lecithal distribution of yolk… found in placental mammals and amphioxus
Mesolecithal – telolecithal distribution of yolk concentrated at the vegetal pole… found in lampreys, bony fish, amphibians
Macrolecithal – telolecithal eggs… found in marine lampreys, cartilaginous fish, reptiles, monotremes

5. Craniate 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 euviviparous
No viviparous turtles, crocodiles, or birds

6. Craniate Eggs Viviparity: Histotrophic vs. placental nourishment
Life in cold blood caecilian clip

7. Craniate 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 sperm
Urodeles (salamanders) may use spermatophores and spermatheca

8. Early 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?

9. Early Development
Let’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?

10. Early Development
Gastrulation is the process in which 3 primitive germ layers are formed
Amphioxous eggs lack yolk and provide a clear picture
Involution of blastula
Resultant opening is the blastopore

11. Early Development
Mesoderm forms from the endoderm and then creates pouches (coelomic or mesodermal)
Lateral/superficial layer becomes somatic mesoderm
Inner/deep layer becomes splanchnic mesoderm
What becomes of the ectoderm?
How about the endoderm?

12. Early 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.

13. Early 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 sac
Epiblast has cells migrating inward that become endoderm
Cells destined to become the mesoderm migrate in between the other 2 “derms”

14. Early Development
What regulates differentiation of these cells into various germs, derms, tissues and organs?
Organizer area
Homeotic genes (sonic hedgehog gene)
Proteins involved in signaling called morphogens
Defective homeotic genes can have severe developmental consequences… ex. spina bifida

15. Early Development
What is the fate of the different “derms”?

16. Germ Layers Placental mammals have varying approaches to gastrulation
However, generally a neural plate forms
Neural folds develop resulting in a neural groove
Neural folds unite forming a neural tube
Process called Neurulation

17. Extraembryonic Membranes
Extraembryonic membranes include:
Yolk sac, amnion, chorion and allantois

18. Extraembryonic Membranes
What does a yolk sac accomplish? How?
Yolk sac is a highly vascular membrane that surrounds the yolk.
Empties into the midgut
Can secrete enzymes to digest yolk
Can serve as respiratory organ in viviparous amphibians/fish
Can absorb nutrients from mother… functions as a simple yolk sac placenta or a “pseudoplacenta”

19. Extraembryonic Membranes
Reptiles and mammals develop inside 2 sacs… Amnion and Chorion:
Amnion surrounds the embryo
Chorion surrounds the amnion and the yolk sac
Important feature that allows eggs to be laid on land (with less dependence on water)
Amniotic fluid surrounds the embryo and is contained by the amnion
Where does this water come from?

20. Extraembryonic Membranes
Allantois is an evagination of the cloaca
Communicates with the inner surface of the chorion forming the chorioallantoic membrane
Reptiles 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

21. Extraembryonic Membranes
Placenta generally is any place embryonic and maternal tissues come together for exchange
More specifically/restricted definition… organ containing highly vascular region of extraembryonic membrane in communication with highly vascular region of maternal tissue

22. Extraembryonic Membranes
Placentas can have a simple communication with maternal tissue… nondeciduous placenta Or
They can have chorionic villi that invade endometrium and pull part of this lining off with birth… deciduous placenta
Villi can be positioned variously on the chorion