Laboratory methods: Which type of embryo is classically used for viral assays, isolation, and propagation in embryonated eggs?

Introduction / Context:
Before widespread cell culture systems, and still today for specific applications, embryonated eggs provide a convenient, sterile, living system to grow and assay viruses. Understanding which eggs are used and why remains relevant for diagnostics, vaccine production, and basic virology labs.

Given Data / Assumptions:

• Embryonated eggs provide multiple inoculation sites (chorioallantoic membrane, allantoic cavity, yolk sac, amniotic cavity).
• Common target viruses include orthomyxoviruses (e.g., influenza), poxviruses, and others.
• The question asks which embryo is “often used” in this context.

Concept / Approach:
Fertilized chicken eggs (usually 9–12 days old) are the standard for many viral assays. They are economical, readily available, and offer compartmentalized environments that support viral growth and visualization of pocks or hemagglutinating virus production.

Step-by-Step Solution:

Verification / Alternative check:
Standard lab manuals and vaccine manufacturing protocols cite 9–11 day embryonated chicken eggs as the workhorse system.

Why Other Options Are Wrong:

• Mouse, Rat, Cat: not used as embryonated egg assay systems; while animal models exist, they are not the classic egg-based platform.

Common Pitfalls:
Confusing animal infection models with egg-based propagation; the question specifically refers to embryos used for viral assays in eggs.

Final Answer:
Chicken