Embryology underwent some very significant changes during the seventeenth century. At the beginning of the century, embryology was descriptive and clearly directed toward morphological goals; by the end of the century, a dynamic, more physiological attitude was apparent, and theories of development derived from an entirely different philosophic base. During this time, English investigators contributed much, some of ephemeral, some of lasting importance to the development of embryology. For this discussion, we will divide the seventeenth century into three overlapping, but generally distinct, periods; and, without pretence of presenting an exhaustive exposition, we will concentrate upon the concepts and directions of change characteristic of each period, with primary reference to those individuals who best reveal the character of seventeenth-century English embryology. An understanding of the characteristics of embryological thought at the beginning of the seventeenth century may enhance appreciation of later developments. During the latter part of the sixteenth century, the study of embryology was, for obvious reasons, most often considered within the province of anatomy and obstetrics. From Bergengario da Capri to Jean Riolan the Younger, study of the fetus was recommended as an adjunct of these subjects, and it required investigation by direct observation, as decreed by the "restorers" of anatomy. Embryonic development was, however, also studied independently of other disciplines by a smaller group of individuals, and the study of chick development by Aldrovandus, Coiter, and Fabricius ab Aquapendente laid the basic groundwork of descriptive embryology. In either case, during the last half of the sixteenth century the attempt of the embryologist to break with the traditions of the past was overt, although consistently unsuccessful. When dealing with the fetus, the investigators of this period were, almost to a man, Galenists influenced to varying degrees by Hippocrates, Aristotle, and Avicenna. Each felt compelled to challenge the immediate authority, and yet their intellectual isolation from the past was incomplete, and their views on embryogeny corresponded with more often than they differed from those of the person they railed against.