Animal Development Chapter 22

What happens when a mother's egg becomes fertilized? Do you have a microscopic baby at this point? What does Ontogeny recapitulates Phylogeny mean?

I. Embryonic Development

A. Cleavage- Cells don't grow, just divide; therefore, cell size decreases

1. series of cell divisions after fertilization
2. mitosis
3. embryo-development of the zygote (one cell)

B. Stages of Cleavage- the number of cells doubles at each division but no growth occurs.

1. Morula-developing embryo is solid ball of cells
2.Blastula-development of hollow center filled with fluid-blastocoel
3. Gastrulation-"horse shoe stage" when blastula reaches several hundred cells

a. cells begin to grow when dividing
b. pinching of cells inward to produce 2 layers-gastrula.

i. blastopore-opening into the gastrula
ii. outer layer-ectoderm
iii. inner layer-endoderm
iv. primitive gut-cavity within the gastrula
v. after 2 layers form, third middle layer is formed-mesoderm.

In gastrulation a second layer is formed by the cells of one side pushing inward, forming an indentation. The mesoderm forms between the outer layer and inner layer.

Ectoderm	Mesoderm	Endoderm
first to form	third to form	second to form
nervous system		lining of the digestive tract
epidermis, sweat glands, hair and nails	bones & muscles	lining of the trachea, bronchi, and lungs
lining of mouth, nostrils and anus	blood, blood vessels, & heart	liver & pancreas
	reproductive and excretory systems	thyroid, parathyroid, & thymus
	inner layer of skin	urinary bladder

C. Growth and Differentiation

1.Differentiation-series of changes that transforms the unspecialized embryonic cells into specialized cells, tissues, and organs
2. Growth-increasing number of cells
3. Click to see pictures of human embryo developing

D. Review of Embryological Events

1. fertilization
2. zygote
3. cleavage
4. growth
5. differentiation

II. Control of Development

A. Nucleus

1. DNA-contains hereditary information.

a. controls cellular activity
b. encodes for the production of cellular proteins

B. Cytoplasm-receives instructions from the DNA in the nucleus.

C. Organizers-certain parts of the developing embryo influence the development of adjacent cells

D. Embryonic induction-process by which an organizer induces another structure to differentiate

IV. Development

A. External in water

1. nourishment is supplied by the yolk stored in the egg
2. no parental care

B. External on land- production of egg and shell, some parental care

1. extra embryonic membranes- 4 membranes outside of the embryo

a. chorion-outermost membrane-aids in gas exchange
b.allantois-exchange of oxygen and carbon dioxide, stores wastes until egg hatches
c. amnion-fluid filled sac that surrounds the embryo -cushion, shock absorber
d. yolk sac- surrounds the yolk- source of food for embryo

example of organism-lizards, birds, and snakes

2.shell- prevents bacteria from entering , porous enough for oxygen and carbon dioxide to exchange, slows rate of evaporation.

C. Internal Development

1. placental mammals-blood vessels of embryo are in close contact with the mother's blood

a. placenta-structure produced by the uterus of the mother which supplies nutrients to the embryo and removes wastes from the embryo.
b. umbilical cord-attaches the embryo to the placenta of the mother in the uterus

2. Non placental mammals- 2 types

a. egg laying mammals -duckbill platypus, spiny anteater
b. marsupials- pouched mammals-kangaroo, opossum

3. marsupials embryo is born in an immature stage, egg laying mammals are not

4. all embryos have mechanisms for absorbing oxygen

Ontogeny is the coarse of development of an organism from fertilized egg to adult; phylogeny is the evolutionary history of a group of organisms. The phrase, originating the 19th century biology by Ernst Haeckel, means that, as an embryo of an advanced organism grows, it will pass through stages that look very much like the adult phase of less advanced organisms. For example, at one point the human embryo has gills and resembles a tadpole. The recapitulation myth, better known as the biogenetic "law", claims that each embryo in its development passes through abbreviated stages that resemble developmental stages of its evolutionary ancestors. Students of biology who have gone to the trouble to memorize this impressive sounding phrase will be disheartened to learn that it has been known to be untrue. It is wrong!

Biology Trivia

1. What determines the sex of the Philippine crocodile? answer

return to Mr. Via's syllabus