How the behaviour of animals evolves by natural selection, towards optimal solutions, highlighting basic principles and processes.
- Introduction and methodology;
- Levels of selection and the Comparative Method;
- Development of behaviour: role of the environment, learning, role of genes;
- ...
Lecture 2- The frog, Xenopus laevis, the African clawed toad
- Fertilisation- 1 cell
- Undergoes cleavage
- Generate a large number of cells
- Forms blastula
- 6 hours after fertilisation-10000 cells
- Rapid cell division
- Cell division stops
- Cells move in respect to each other- gastrulation
- 10 hours
- Move in coordinated fashion to generate embryo
- Organogensis- generate different organs that make tadpole
,Blastula- early stage embryo consisting of a sphere of cells surrounding inner fluid filled cavity blastocoel in
frog and fish.
Blastocyst- mammalian blastula. the blastocoel is expanded and the inner cell mass is positioned on one
side of the ring of trophoblast cells.
Blastocoel- fluid filled cavity that forms an animal hemisphere of early amphibian and echinoderm embryos,
or between the epiblast and hypoblast of avian, reptilian and mammalian blastoderm-stage embryos.
Blastopore- the invagination point where gastrulation begins. In deuterostomes this marks the site of the
anus. In protostomes this marks the site of the mouth.
Blastoderm- single layer of embryonic epithelial tissue that makes up the blastula.
Blastodisc - small region at the animal pole of the telolecithal eggs of fish and chicks, containing the yolk
free cytoplasm where cleavage can occur and that gives rise to the embryo. Following cleavage the
blastodisc becomes the blastoderm.
Blastomere- a cleavage-stage cell resulting from mitosis.
,Scanning electron micrographs of early Xenopus embryos
Cleavage furrow- generate more cells
Positions of initial cleavage furrows
(A)
- Grey cresent- top of figure- animal pole
- Bottom- vegetal pole- primary access down through egg- animal to vegetal
- Yolk more concentrated at vegetal pole
- Cell division starts from animal pole
- Large cells take a while for cleavage furrow to pass through whole single cell to generate 2 cells
- Starts with animal pole
- Yolk in vegetal pole inhibits cytokinesis - cell division
(B)
, - Cell division occurs at right angles to first cell division from animal pole
- 2nd cell division starts before 1st finished
- 3rd cell division takes place- equatorially
- Right angle to first 2 cell divisions
- Divide embryo across
- Displaced towards animal pole due to inhibition by yolk
- Animal pole cells are smaller than vegetal cells
- Subsequent cells divisions right angles to divisions
- Carry on rapidly
(F)
- Blastomeres dividing
- Large number of cells
- Gradient in the size of the cells but divide at same rate
- Smaller- animal pole
- Larger- vegetal pole
(H)
- 128 cell stage- Fluid filled cavity— blastocoel forms
• Functions: permit cell migration during gastrulation and prevent cells underneath interacting
prematurely with cells above it
- All cells are known as blastomeres in blastula
- Gradient of cell sizes
- Cleavage burrows right angles to each other- large cell- large amount DNA replicated
- Cell divisions are 30 mins apart
- Bacteria- smaller cells- smaller genome-20 mins cells division
- Rapid rate of cell division
SOCRATIVE
Diameter of zygote- 2mm
Frog is a toad
Diameter of blastula after cleavage - 2mm - no growth from zygote to blastula- single cell that is
organised- development not growth
20,000 genes in human genome
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