Early Embryology / Animal Development Understanding the amniotic egg © 2018 Pearson Education, Inc.

Fertilization © 2018 Pearson Education, Inc.

27.10 Cleavage produces a blastula from the zygote Cleavage is a rapid series of ______ ___________that produces more cells, smaller cells, and a fluid-filled cavity, the blastocoel, in an embryo called a blastula. How does the reduction of cell size during cleavage increase oxygen supply to the cells’ mitochondria? (Hint: Consider the effect of surface area.) Checkpoint Question Response Smaller cells have a greater plasma membrane surface area relative to cellular volume, which facilitates diffusion of oxygen from the environment to the cell’s cytoplasm. Student Misconceptions and Concerns The descriptions of basic development address questions that students may have never thought to ask. How do we get so many cells (trillions!) in our adult bodies? How do basic tissues, organs, and organ systems form? What mechanisms permit the coordinated development of the body? Before beginning this chapter, consider challenging your students with some of these fundamental questions: How did you get a heart, a brain, and ears? What determined when you were ready to be born? How did you get nutrients and oxygen before you could eat or breathe? (27.9–27.14) Teaching Tips Cleavage is largely a process of subdivision with no growth. This process is a bit like cutting up a pie into pieces. With every division, more pieces are produced, but the pieces are smaller and the pie itself does not increase in size. Similarly, cleavage increases the number of cells while decreasing their size. (27.10) Cleavage in humans is a very slow process, taking up to 12 hours for each division. (27.10) Cleavage results in an uneven distribution of cytoplasmic elements into the daughter blastomeres (embryonic cells produced by cleavage). Some students might benefit from this simple analogy: Imagine baking a pie that is filled with one can of apple pie filling and one can of cherry pie filling. The contents of each can are poured into opposite ends of the pie and are not mixed. When the pie is served, some pieces will contain only apples, some only cherries, and some combinations of both. This disproportionate division of pie contents is like the disproportionate division of cytoplasmic elements during cleavage. (27.10)

The initial stages of embryology are the same in all animals. Sperm EMBRYONIC DEVELOPMENT Zygote Adult frog Egg The initial stages of embryology are the same in all animals. FERTILIZATION CLEAVAGE Blastocoel Metamorphosis GASTRULATION Blastula Figure 27.10 Development stages of a frog ORGAN FORMATION Tadpole Gastrula Embryo

Video: Sea Urchin Embryonic Development

27.11 Gastrulation produces a three-layered embryo During gastrulation, cells ____________ to new locations, a rudimentary ____________cavity forms, and the basic body plan of three layers is established with ectoderm outside—becoming skin and nervous systems, endoderm inside—becoming digestive tract, and mesoderm in the middle—becoming muscle and bone. Student Misconceptions and Concerns The descriptions of basic development address questions that students may have never thought to ask. How do we get so many cells (trillions!) in our adult bodies? How do basic tissues, organs, and organ systems form? What mechanisms permit the coordinated development of the body? Before beginning this chapter, consider challenging your students with some of these fundamental questions: How did you get a heart, a brain, and ears? What determined when you were ready to be born? How did you get nutrients and oxygen before you could eat or breathe? (27.9–27.14) Teaching Tips You might wish to reflect on the somewhat famous quote of Lewis Wolpert, who in 1986 said, “It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life.” The development and arrangement of the basic embryonic layers (ectoderm: skin and nervous system; mesoderm: muscle and bone; and endoderm: digestive tract) establish the basic body plan. (27.11) Gastrulation establishes the basic body plan by positioning the future systems of the body in relationship to each other. This essential element of gastrulation is a crucial event that is a prerequisite for later developmental events (and thus is the basis of the quote directly above). (27.11) The colors used in the text to depict the three layers of embryonic tissue are standards used in embryology. Blue represents ectoderm and its derivatives, red represents mesoderm and its derivatives, and yellow represents endoderm and its derivatives. (27.11) Neural crest cells are often called the fourth layer of embryonic tissue because these cells give rise to a variety of tissues in diverse locations in the body. Neural crest cells and their derivatives are not addressed in Chapter 27. (27.11)

Table 27.11 Derivatives of the three embryonic tissue layers