They begin moving to new positions in the process of gastrulation, which brings the germ layers into positions appropriate for their specific fates. You will see how these movements are choreographed in the sea urchin, fruit fly, and chick in Chapters 6 through 10. With the germ layers in position, the trip enters a phase of dynamic interactions known as organogenesis. Cells in the embryo relay information back and forth, and these interactions lead to organ differentiation. In Chapters 6 through 11 you will glimpse this final leg of the embryonic journey as you raise sea urchin larvae, dissect and experiment with imaginal discs of fruit fly larvae, and test the responses of one of the earliest products of organogenesis in the chick, the embryonic heart.
Development is a long journey that extends well beyond the event of hatching or birth.
One of the fascinating side roads is that of regeneration. Few organisms are capable of such impressive feats of regeneration as those seen in planaria. Organisms such as sea urchins and amphibians, which must entrust their poorly protected embryos to the open wilds, can be particularly vulnerable, and therefore make sensitive barometers of environmental hazards. In Chapter 7, the hazards of ultraviolet radiation, an ever increasing threat, are examined using the delicate sea urchin embryo. In Chapter 14 we end our study of the developmental journey by exploring the woods and streams for evidence of threats to embryos of our cool and thin-skinned amphibian relatives. .