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HEREDITY AND DEVELOPMENT: SECOND EDITION 227 CONCEPTS IN EMBRYOLOGY INTRODUCTION Embryology is the branch of biology dealing with the events associated with the formation of an adult individual from a fertilized ovum. The magnitude of the events during this period can be inferred from a consideration of the dif- ferences between a fertilized ovum and the adult of an animal such as the frog. The fertilized ovum consists of a single cell, the frog of many billions; and the differences are more profound than mere cell number. A billion frog eggs, no matter how arranged, do not constitute a frog. The cells derived from the single-cell zygote differentiate along many separate pathways between the fertilized ovum and the adult. Some form muscle cells, others neurons, and still others the specialized cells of the liver, stomach, kidney, endocrine glands, and gonads. All the diverse cell types of the adult are derived from a single cell, the fertilized ovum. There are two man processes involved in development. One is a tremen- dous increase in cell number. This is accomplished by mitotic division of already existing cells, or as Virchow expressed it, omnis cellula e cellula. The increase in cell number involves an increase in total mass and this, in turn, necessitates the utilization of food from some extraneous source. The second process is differentiation. The term probably suggests the nature of the process. During the course of development, the cells change in structure and function: they become different from their earlier forms and from one another. Our aim in the study of embryology shall be to ask and attempt to
HEREDITY AND DEVELOPMENT: SECOND EDITION 228 answer some of the pertinent questions concerning development. The âwhysâ and âhowsâ of development are among the most exciting questions that a biol- ogist can ask. We must begin our study of embryology, however, with a brief description of early development. It is necessary that we have this back- ground before we can ask intelligent questions about the dynamics of the embryological processes. The frog will be used as a type since more is known about its development than about that of any other animal.