Clay models have proved to be useful teaching aids for many topics in biology that depend on three-dimensional reasoning. Students studying embryonic development struggle to mentally reconstruct the three-dimensional structure of embryos and larvae by observing prepared slides of cross-sectional slices. Students who build clay models of embryos and slice them into cross sections, however, have a much easier time interpreting the slices of real embryos and gain a deeper understanding of development in three dimensions.
We developed a low-cost, high-impact activity that helps students make tangible connections between cross sections and whole embryos or animals. It has proved effective in a university-level developmental biology course, but it would be suitable for high school or middle school courses.
Students often struggle with the abstract concepts of body axes (dorsal-ventral, anterior-posterior, medial-lateral) and anatomical planes (transverse, sagittal, and horizontal). An even bigger challenge is constructing three-dimensional mental models of embryos on the basis of two-dimensional cross sections, like textbook figures and prepared microscope slides of embryonic slices. The following method complements activities that exploit clay models of early embryonic development (Kleiner, 2000; Gilbert, 2003; Ting Chowning et al., 2008).
Blue, red, yellow, and green clay or Play-Doh®®
Rolling pins (e.g., 10-cm lengths of dowel)
Stiff wire (∼∼20-cm lengths)
As illustrated in the figures, students construct idealized vertebrate embryos that have undergone organogenesis, corresponding roughly to Xenopus embryos at Stages 45 through 50, zebrafish embryos between 24 and 48 hours postfertilization, or chick embyros at Stage 17––18. Following convention, blue represents ectoderm-derived tissues, red represents mesoderm-derived tissues, and yellow represents endoderm-derived tissues; in addition, green can be used to represent ectoderm-derived neural tissue.
Figure 1 shows the construction of the models. Students use dowels to roll out sheets of blue ectoderm to represent skin and red mesoderm to represent somites, as well as rolling long cylinders representing spinal cord (green neural ectoderm), notochord (red mesoderm), and gut (yellow endoderm). They can also shape internal organs (e.g., heart from red mesoderm and liver from yellow endoderm). The model is assembled dorsal side down and finished by bringing the long edges of the skin together and pinching them closed at the ventral midline.
Figure 2 shows the results of transverse and sagittal sectioning. Students carefully slice through their models with stiff, thin wire. To match the conventional arrangement of transverse slices of preserved embryos on prepared microscope slides, students start at the anterior and lay out the slices left to right.
Confronted with prepared slides of a series of sections, students often struggle to conceptualize the whole organism in three dimensions. Building and slicing their own models before examining slides, they have much more success.