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Chapter 34: Problem 31

What are the three phases of embryonic development in chronological order? a. blastula ? gastrula ? cleavage b. blastula ? cleavage ? gastrula c. cleavage ? gastrula ? blastula d. cleavage ? blastula ? gastrula

Short Answer

Expert verified
The correct sequence is option d: cleavage 鈫� blastula 鈫� gastrula.

Step by step solution

01

Understand the Terms

The terms cleavage, blastula, and gastrula represent different stages in embryonic development. Cleavage is the initial rapid cell division after fertilization. The blastula stage follows cleavage and is a hollow ball of cells. The gastrula stage follows the blastula and involves the formation of the three germ layers.
02

Identify the First Phase

Recognize that the first phase is cleavage, which is the period of rapid cell division after a zygote is formed.
03

Identify the Second Phase

After cleavage, the developing embryo transitions into the blastula phase, characterized by the formation of a hollow ball of cells.
04

Identify the Third Phase

The final phase is the gastrula phase, which involves significant cell movements to form the three germ layers.
05

Match the Sequence with the Answer Choices

Given the correct sequence: cleavage, blastula, gastrula, the correct answer is option d: cleavage 鈫� blastula 鈫� gastrula.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Cleavage
Cleavage is the opening phase of embryonic development. Right after fertilization, the zygote undergoes **rapid cell divisions**. These divisions are called cleavage. What鈥檚 important here is:
  • Cleavage leads to many smaller cells without an increase in the overall size of the embryo.
  • This process is different from regular cell division because the zygote doesn't grow in size but is divided into smaller units called blastomeres.
  • Each division happens swiftly, creating a solid ball of cells known as the morula.
Breaking up the zygote into plenty of cells sets the stage for the next phases. Each of these cells will contribute to forming a complex organism. Without cleavage, the following stages wouldn't be possible.
Blastula
After numerous cleavages, the morula forms into the **blastula**. The blastula stage is crucial because it organizes the cells into a **hollow ball structure**:
  • Inside the blastula, you'll find a fluid-filled cavity called the 'blastocoel.'
  • This stage ensures that cells are now organized into at least two layers.
  • Each of the resulting cells is known as a blastomere, and they are pivotal for future differentiation.
The symmetry and organization seen in the blastula are imperative. They prepare the embryo for even more complex arrangements and differentiations needed in the following gastrula stage.
Gastrula
Moving forward from the blastula, the **gastrula** stage unfolds. This phase witnesses extensive cell migrations and re-arrangements:
  • The blastula cells start to move inward, forming a deep cavity called the 'archenteron'.
  • This inward movement creates distinct cell layers: the ectoderm, mesoderm, and endoderm. These germ layers later differentiate into various tissues and organs.
  • The gastrula stage establishes the body plan of the emerging embryo, laying down a foundation for organ development.
The development during the gastrula stage is foundational because each germ layer will give rise to specific tissues and organs, crucial for forming a complex organism.

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Most popular questions from this chapter

Discuss the positive feedback mechanisms by LH and FSH during the follicular and ovulation phase of the ovarian cycle. a. The stimulation of the anterior pituitary by GHRH secretes the hormones LH and FSH. The increasing amounts of these hormones stimulate several follicles in the ovary to start growing, but only one of these matures to release the egg. b. The stimulation of the anterior pituitary by GnRH secretes the hormones LH and TSH. The increasing amounts of these hormones stimulate several follicles in the ovary to start growing, but only one of these matures to release the egg. c. The stimulation of the anterior pituitary by GnRH triggers the secretion of the hormones LH and FSH. The increasing amounts of these hormones stimulate several follicles in the ovary to start growing, but only one of these matures to release the egg. d. The stimulation of the anterior pituitary by GHRH secretes the hormones LH and TSH. The increasing amounts of these hormones stimulate several follicles in the ovary to start growing, but only one of these matures to release the egg.

What stimulates Leydig cells in the testes to produce testosterone? a. FSH b. LH c. inhibin d. estrogen

Which method produces genetically-unique offspring? a. parthenogenesis b. budding c. fragmentation d. sexual reproduction

Explain what the mesoderm is and what it eventually differentiates into. a. The mesoderm, located on either side of the vertebrate neural tube, develops into various connective tissues. It is reorganized into groups of cells called somites, which develop into facial cartilage, ribs, and lungs. b. The mesoderm, located at the border of the neural plate and the non-neural ectoderm, develops into various connective tissues such as the ribs, lungs, segmental muscle, and the notochord, which forms the central axis of the animal body. c. The mesoderm, located on either side of the vertebrate neural tube, develops into various connective tissues. It is reorganized into groups of cells called somites, which develop into ribs, lungs, segmental muscle, and the notochord. d. The mesoderm, located at the border of the neural plate and the non-neural ectoderm, develops into various connective tissues such as the facial cartilage, ribs, and lungs.

Which germ layer forms the skin cells? a. endoderm b. ectoderm c. mesoderm d. trophoblast
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