Developmental

Question 1

Gamete Formation

Answer 1

gametes => mature haploid male or female germ cell

Question 2

What is an egg?

Answer 2

• Egg
  An organic vessel where an embryo develops (colloquial terminology)
  An ovum
• Ovum
  The female reproductive or germ cell
• Eggs vary significantly among taxa
• Human egg is approx. 100 microns
  Width of a human hair
  Huge
• Eggs has components of a normal somatic cell
• Eggs also have yolk (provides energy during development)
• Eggs have a polarity
  Animal pole has most of the cytoplasm (and the nucleus)
  Vegetal pole has most of the yolk

Question 3

Isolecithal

Answer 3

• Very little yolk, evenly distributed through egg

- Typical of placental mammals (also echinoderms, tunicates, cephalochordates, molluscs)

Question 4

Mesolecithal

Answer 4

• Moderate amount of yolk concentrated at vegetal pole

- Typical of amphibians

Question 5

Telolecithal

Answer 5

• Abundance of yolk densely concentrated at vegetal pole

- Typical of birds, reptiles, fish, monotremes, some amphibians

Question 6

Centrolecithal

Answer 6

• Large centrally located mass of yolk

- Typical of arthropods

Question 7

Amount of yolk is associated with developmental patterns

Answer 7

• Lots of yolk (telolecithal eggs), young exhibit direct development
  Goes straight from embryo to miniature adult
• Little yolk (isolecithal, mesolecithal eggs) young exhibit indirect development
  Passes through larval stage capable of feeding itself
  Undergoes metamorphosis to reach adult stage
  Exception: mammals (direct development)

Question 8

What is fertilization

Answer 8

• Fertilization => gametes unite to form a zygote

- Zygote => a diploid cell resulting from fusion of male and female gametes (i.e. a fertilized egg)

Question 9

What is cleavage?

Answer 9

• embryo divided repeatedly without growth (essentially skips G-phase of mitosis)
• single large egg cell becomes many smaller cells called blastomeres
• by the end of cleavage, the zygote is called blastula
• cleavage subdivides the zygote to form a blastula
• in most animals the cells are formed around a fluid-filled cavity called a blastocoel
• All multicellular animals go through blastulation. Most continue to gastrulation

Question 10

What is a blastula?

Answer 10

a fluid filled ball of cells

Question 11

Amount of yolk is associated with cleavage patterns

Answer 11

• cell division occurs more easily in cytoplasm than yolk
• two types of cleavage
  holoblastic
  meroblastic

Question 12

What is holoblastic?

Answer 12

complete and approximately equal divisions of cells

Question 13

What is meroblastic?

Answer 13

restricted to a small area of egg

Question 14

What are the directions of cleavage?

Answer 14

Spiral cleavage

Radial cleavage

Question 15

What is gastrulation?

Answer 15

• converts the spherical blastula into two or three layered embryo
• the layers are called germ layers
  The blastula has 1 germ layer
  The gastrula has 2 or 3

Question 16

Gastrulation (1)

Answer 16

• one side of the blastula bends inwards in a process called invagination
• the internal pouch that is formed is the gut cavity (also called archenteron or gastrocoel)
• the opening to the gut cavity is called the blastopore

Question 17

Gastrulation (2)

Answer 17

• outer layer of cells (lining blastocoel) = ectoderm
• inner layer of cells (lining gut) = endoderm
• when the gut opens only at the blastopore it is called a blind gut

Question 18

Gastrulation (3)

Answer 18

• most animals have a complete gut with a second opening, the anus
• the blastopore becomes the mouth in some organisms and the anus in others
• this difference differentiates Protostomes and Deuterostomes

Question 19

Gastrulation (4)

Answer 19

• in many animals, a third layer forms = mesoderm
• the mesoderm is formed from the endoderm
• coelom = cavity completely surrounded by mesoderm

Question 20

How many layers?

Answer 20

• some animals stop at the blastula stage
  One germ layer
• some animals develop two germ layers
  diploblastic
• most animals develop three germ layers
  triploblastic

Question 21

What is Organogenesis?

Answer 21

• organs develop from specific germ layers
• first event in organogenesis is formation of the nervous system
  Formed from the ectoderm
• heart is the first functional organ
  Formed from the mesoderm
  Starts beating on day 2 of development in the chick embryo

Question 22

What is growth?

Answer 22

• final phase

- longest phase in animal development

Question 23

Categorizing Metazoans

Answer 23

1) Number of germ layers (diploblastic, triploblastic)
2) Presence and formation of coelom
3) Cleavage pattern
4) Body symmetry
5) Mechanism of development