11.12.2 Cleavage, Gastrulation, and Organogenesis: A Closer Look

Question 1

Cleavage, Gastrulation, and Organogenesis: A Closer Look

Answer 1

• Following fertilization, there are three major stages of development:

1. During cleavage, the egg goes through a rapid series of mitotic divisions and other changes, resulting in a hollow ball of cells called the blastula.
2. During gastrulation, the cells of the blastula rearrange, resulting in a three-layered embryo.
3. During organogenesis, the cells of the three layers (ectoderm, mesoderm, and endoderm) differentiate and begin to form tissues and organs.

Question 2

note

Answer 2

• Review: During fertilization, the egg and the sperm nuclei fuse to form a diploid zygote, a large one-celled structure.
• After fertilization, three major developmental stages take place:
• Cleavage is characterized by a rapid series of mitotic
  divisions. Cleavage divides the cytoplasm of the zygote into many smaller cells, each of which is called a blastomere. Eventually cleavage produces a solid ball of cells called the morula.
• With the exception of mammals, the zygotes of most animals have a polarity. The presence of yolk (nutrients stored in the egg) affects the pattern of cleavage. The yolk is most concentrated at one pole – the vegetal pole, and least concentrated at the opposite pole – the animal pole.
• A fluid filled cavity called the blastocoel forms within the morula and the hollow ball of cells is called the blastula.

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Question 3

organogenesis

Answer 3

• During the process of organogenesis, the three germ layers develop into rudimentary organs.
• The ectoderm forms skin, nervous system, and the cornea and lens of the eye, among other organs and tissues.
• The mesoderm forms the notochord – muscles, skeleton, circulatory system, respiratory system, and reproductive system. The notochord is a cartilaginous structure that establishes a dorsal axis in a developing embryo and may later take part in the formation of the vertebrae.
• The endoderm forms the inner linings of many organs such as the digestive tract, liver, pancreas and thyroid.

Question 4

gastrulation

Answer 4

• During gastrulation, cells of the blastula rearrange to form a three-layered embryo with ectoderm, mesoderm, and endoderm germ layers.
• The illustration on the left shows key events that take place during gastrulation. The colors blue, red, and yellow represent ectoderm, mesoderm, and endoderm, respectively. The first sign of gastrulation is a small infolding, the dorsal lip of the blastopore, at the vegetal pole of the blastula. The infolding forms because cells have started to migrate inward. cells of the vegetal pole continue to roll inward over the dorsal lip of the blastopore, eventually forming the endoderm and mesoderm. Endoderm cells form the archenterons (future digestive tube). Cells of the animal pole migrate over the embryo’s outer surface, forming the ectoderm.
• At the end of gastrulation, the gastrula has a functioning digestive tube (derived from endoderm) complete with mouth and anus. The three germ layers – ectoderm, mesoderm, and endoderm – are in place.

Question 5

Which of the following statements regarding gastrulation in sea urchins is not correct?

Answer 5

• The open end of the early archenteron, the blastopore, will become the mouth.

Question 6

What is the cavity in a blastula called?

Answer 6

• blastocoel

Question 7

What is the series of rapid mitotic divisions that occur right after fertilization called?

Answer 7

• cleavage

Question 8

Gastrulation is

Answer 8

• the migration of cells to produce three primary germ layers.

Question 9

Which statement regarding the archenteron of the frog embryo is correct?

Answer 9

• The archenteron is fully lined with endoderm.

Question 10

After the first several cell divisions, continued cleavage produces a

Answer 10

• morula