MBRYOLOGY studies the succession of transformations undergone by the
fertilized E egg in the formation of a new individual. Development of
the embryo is directed by morphogenetic mechanisms ruled by a strict
chronology. Survival of the egg, its transport in the genital tract, and
the adaptation of the maternal orga- nism to its presence are controlled
by hormonal actions. Knowledge of these subjects is proving to be
increasingly important for the medical practitioner. Such information
helps to explain anatomic correlations; organ relation- ships also
illuminate the etiology of numerous pathologic conditions. Disturbances
of prenatal development engender congenital malformations and constitute
an important cause of perinatal mortality and postnatal morbidity. Our
goal in preparing Volume I was to introduce the student to the complex
pheno- mena of embryonic development in a clear and direct way. We tried
to present an overall view of the simultaneous nature of the multiple
and rapid events in embryogenesis. Also, we thought it appropriate to
treat certain topics in depth, becallse of their difficulty, or their
physiological or pathological implications. Thus, gastrulation and
formation of the body shape, including flexion, which are par- ticularly
dynamic processes, can best be understood by following their sequences
in detail. Special attention '}Jas also devoted to the placenta, since
it assures the functions of nutrition, respiration, and excretion. It is
also an endocrine gland upon which the hor- monal equilibrium of
pregnancy depends.
