Lecture 7

Question 1

What is a gamete

Answer 1

mature haploid male or female germ cell (i.e. egg and sperm)

Question 2

What is an egg?

Answer 2

• An organic vessel where an embryo develops
(colloquial terminology)
• An ovum (scientific terminology)

Question 3

Ovum

Answer 3

the female reproductive or germ cell

Question 4

Egg characteristics

Answer 4

Eggs has components of a normal somatic cell (e.g. cytoplasm, nucleus, organelles)

• 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 5

placement of yolk in egg

Answer 5

Isolecithal
Mesolecithal
Telolecithal
Centrolecithal

Question 6

Isolecithal:

Answer 6

very little yolk, evenly distributed through egg
Typical of placental mammals
*Also echinoderms, tunicates, cephalochordates, molluscs

Question 7

Mesolecithal:

Answer 7

moderate amount of yolk concentrated at vegetal pole

Typical of amphibians

Question 8

Telolecithal:

Answer 8

abundance of yolk densely concentrated at vegetal pole

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

Question 9

Centrolecithal:

Answer 9

large centrally located mass of yolk

Typical of arthropods (e.g. insects)

Question 10

General rule of amount of yolk

Answer 10

Lots of yolk (e.g. telolecithal eggs), young exhibit direct development
• Goes straight from embryo to miniature adult

Little yolk (e.g. isolecithal, mesolecithal eggs) young exhibit indirect development
• Passes through larval stage capable of feeding itself • Undergoes metamorphosis to reach adult stage
• e.g. amphibians, echinoderms, tunicates, cephalochordates, molluscs

Question 11

Why are mammals an exception to amount of yolk rule

Answer 11

because they don’t need a larval stage due to mother giving so much energy , ie direct development

Question 12

Cleavage

Answer 12

• Embryo divides 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 a blastula

Question 13

Cell division occurs more easily in _____ than yolk

Answer 13

cytoplasm

Question 14

Two types of cleavage:

Answer 14

• Holoblastic: complete and approximately equal divisions of cells
• Meroblastic: restricted to a small area of egg

Question 15

Direction of cleavage

Answer 15

• Spiral cleavage

* Radial cleavage

Question 16

Where are cells formed in most animals

Answer 16

around a fluid-filled cavity called a blastocoel

Question 17

True or False: most animals go through blastulation

Answer 17

FALSE! ALL ANIMALS go through blastulation, most go through gastrulation

Question 18

Gastrulation

Answer 18

• Converts the spherical blastula into a two- or three- layered embryo
• The layers are called germ layers

Question 19

How many germ layers does the blastula and gastrula have?

Answer 19

the blastula has 1 germ layer

the gastrula has 2 or 3

Question 20

Gastrulation process

Answer 20

• One side of the blastula bends inward 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
• 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
• 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
7.1 Key Events in Animal Development
• In many animals, a third layer forms = mesoderm
• The mesoderm is formed from the endoderm
• Coelom = cavity completely surrounded by mesoderm

Question 21

one germ layer

Answer 21

Some animals stop at the blastula stage (e.g. some sponges)

Question 22

Two germ layers

Answer 22

Some animals develop two germ layers (e.g. some sponges, sea anemones and comb jellies)
• diploblastic

Question 23

Three germ layers

Answer 23

Most animals develop three germ layers (e.g. everyone else!)
• triploblastic

Question 24

Organogenesis

Answer 24

• 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

Question 25

Growth

Answer 25

• Longest phase in animal development
ex
• The human fetus grows from the size of a single poppy seed (4 weeks) to an 8lb jackfruit (40 weeks)
• Organogenesis takes place in the first 8 weeks

Question 26

Categorizing Metazoans

Answer 26

1. Number of germ layers (diploblastic, triploblastic)
2. Presence and formation of coelom (acoelomate, pseudocoelomate, coelomate) and (next: schizocoely, enterocoely)
3. Cleavage pattern (radial, spiral)
4. Body symmetry (e.g. spherical, radial,
   bilateral)
5. Mechanism of development (mosaic, regulative)