Lab 20: Descent With Modification and Embryonic Development is one of a series of lab investigations for the middle school student from the book, “Argument-Driven Inquiry in Life Science: Lab Investigations for Grades 6-8”. Students are introduced to the idea of descent with modification and read about adaptations and homologous structures such as limbs in mammals. They then look at images of embryo development in eight different vertebrates and design a simple investigation to answer the question of whether animal embryonic development supports or refutes the theory of descent with modification. Students then engage in argumentation with other groups in the classroom based on the data that they have collected from the embryo pictures and their analysis of the data. “Checkout Questions” are provided to facilitate student reflection on what was learned. Students are assigned a short investigation report to finish processing their experience. Significant background information is provided for teachers and to a lesser degree in the student hand-out in the introduction. The NGSS standards addressed in the lesson are also included in the teacher’s notes.