ABSTRACT
I. INTRODUCTION Our thoughts about embryogenesis historically have centered on two questions. First, what is the substance from which a new individual is made? The fertilized egg has always been recognized as the formative substance of such organisms as insects, fish, reptiles, amphibians, and birds, since these species have macroscopic eggs that develop outside the maternal body. Harvey deduced in the seventeenth century that an egg was also the source of the internally developing mammalian embryo, a deduction that was confirmed microscopically in the nineteenth century. Sperm were discovered by Leeuwenhoek with the primitive microscopes of the seventeenth century and were verified in the early nineteenth century as the male reproductive principle that fertilized the egg. With the formulation of the cell theory in 1839, the egg and sperm were recognized as cells that unite to form a zygote that divides repeatedly to create a multicellular embryo.
