But, strangely enough, the course of development in any higher organism is not straightforward, but devious. The bird, as well as all higher animals, acquires gill-clefts and other rudimentary structures not adapted to its condition of life. Most of the rudimentary organs are transitory and bear testimony, as hereditary survivals, to the line of ancestry. They are clues by means of which phases in the evolution of animal life may be deciphered.
Bearing in mind these shifting changes, one begins to see why the adult structures of animals are so difficult to understand. They are not only complex they are also greatly modified. The adult condition of any organ or tissue represents the last step in a series of gradually acquired, modifications, and is, therefore, the farthest departure from that which is ancestral and archetypal. But in the process of formation all the simpler conditions are exhibited. If, therefore, we wish to understand an organ or an animal we must follow its development, and see it in simpler conditions, before the great modifications have been added.
The tracing of the stages whereby cells merge into tissues, tissues into organs, and how the organs by combinations build up the body, is embryology. It has become one of the richest and most suggestive of the biological sciences in furnishing clues to the past history of animals and throwing light on their relationships.
It is the purpose of this paper to trace in a summary way the rise of that interesting division of biological science, pointing out its epochs, and telling something about the men who laid its foundations, the discussion being limited to the animal side, with no attempt to represent the rise of plant embryology. That we can 'read all history in the lives of a few great men' is essentially true in reference to the progress of embryology. There are many individual workers, each contributing his share, but the ideas of the science are molded into effective form only in the minds of the leaders. In this group of 'the leaders,' Von Baer stands as a monumental figure, at the parting of the ways between the new and the old—the sane thinker, the great observer.
The story of the rise of embryology can, for convenience, be divided into five periods each marked by an advance in general knowledge. These are: (1) the period of Harvey and Malpighi; (2) the period of Wolff; (3) the period of Von Baer; (4) the period from Von Baer to Balfour; and (5) the period of Balfour with an indication of present tendencies.
The Period of Harvey and Malpighi.
In General.—The conventional way of looking at the rise of embryology has been derived mainly through the channels of German scholarship. But there is reason to depart from the traditional aspect
