Once a sperm and egg form a diploid, single cell zygote, it subdivides rapidly into many tiny blastomeres, eventually forming a morula. This becomes a blastula and then a gastrula. Cleavage patterns (such as protostome and deuterostome) can differ depending on species. Bird embryos contain a lot of yolk, which the cleavage furrow cannot cross. This leads to slightly different development of the blastula (called a blastoderm or blastodisc), but the gastrula still contains three germ layers (the ectoderm, the mesoderm, and the endoderm). Extraembryonic membranes occur in species that develop via an egg rather than in the uterus of the mother and consist of a shell, chorion, yolk sac, and allantois. In mammals, a placenta and umbilical cord form and allow for nutrient and waste exchange.
The first phase
of embryological development is cleavage. During this process, the
embryo undergoes a series of cell divisions but little overall
During the next
stage, a hollow ball of cells called a blastula forms. A
fluid-filled blastocoel is within the blastula.
relocate and differentiate to form a gastrula. The gastrula consists
of three germ layers: the ectoderm, mesoderm and endoderm. The
following organs and systems develop from each layer:
epidermis, hair, nails, nervous system, cornea and lens of the eye,
circulatory system, musculoskeletal system, excretory system and
most of the reproductive system.
Endoderm linings of the GI and respiratory tracts; pancreas,
liver and gall bladder; thymus; thyroid gland
is the formation of organs. During this process, the cells
differentiate. The notochord and neural tube form early in chordate
refers to the influence of one group of cells on the development of
another group of cells. This process is mediated by diffusible
Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.
The book and CD package include two full length practice exams that are presented with all questions answered and explained. The author also presents detailed reviews of all test topics and includes multiple-choice and free-response questions with answers at the end of each topic's chapter.
This book includes a review of the AP Biology exam format and scoring, proven strategies for answering multiple-choice questions, and hints for tackling the essay questions. The practice tests include answers and explanations and there is also proven test-taking strategies for both the multiple choice and essay parts.