ZOO: DSC-354 Practical - IV
Unit 4: Developmental Biology
1. Developmental Stages of Frog
The frog is a classic model for studying vertebrate development through unequal holoblastic cleavage.
Identification Features (Permanent Slides/Sections)
- Cleavage Stages: Observation of the division of the zygote into smaller blastomeres. Note the difference in size between micromeres (animal pole) and macromeres (vegetal pole).
- Blastula: Characterized by the presence of a fluid-filled cavity called the blastocoel, typically shifted toward the animal pole.
- Gastrula: Identified by the formation of the blastopore and the movement of cells (epiboly and emboly) to form germ layers. Look for the yolk plug stage.
2. Developmental Stages of Chick (Whole Mounts)
Chick embryos are studied using whole mounts to observe the formation of the body axis and organ systems.
Key Hours of Incubation
3. Life Cycle of Drosophila melanogaster
Drosophila (fruit fly) is studied through audio-visual aids to understand holometabolous metamorphosis.
Stages in the Life Cycle
- Egg: Small, oval, with two filaments (respiratory horns) at one end.
- Larva: Three larval instars; they are vermiform (maggot-like) and active feeders.
- Pupa: The stationary stage where tissues are reorganized (histolysis and histogenesis).
- Adult (Imago): The winged reproductive stage.
[Image of the life cycle of Drosophila melanogaster]
Exam Focus: Practical Identification & Identification Keys
Practical Tip: When identifying chick whole mounts, always count the somites. The number of somite pairs is the most accurate way to verify the age of the embryo during the first two days of incubation.
Frequently Asked Questions (FAQs)
- Q: What is the significance of the Primitive Streak in chick development?
A: It is the site where cells migrate inward to form the mesoderm and endoderm, analogous to the blastopore in frogs.
- Q: How can you tell a 48-hour chick embryo from a 72-hour one?
A: A 48-hour embryo shows a cranial flexure (head twist), while a 72-hour embryo shows both cranial and cervical flexures, along with distinct limb buds.
- Q: Why is Drosophila called a holometabolous insect?
A: Because it undergoes a "complete" metamorphosis including a pupal stage.
Common Identification Error: Do not confuse the blastocoel (in the blastula stage) with the archenteron (primitive gut in the gastrula stage). The archenteron is formed as the blastocoel is displaced.