EMBRYOLOGY 1 79 



of such machines while at rest have a sort of potentiality of motion in 

 them, and when any external force puts the first of them in motion, im- 

 mediately the next is moved in actuality." ^ This cannot be called a refined 

 physicochemical analysis of development in the modern sense, but it marked 

 a beginning in the right direction. Aristotle also compared the growth of 

 an embryo to the effect which rennet has on milk and, likewise, to the 

 growth of the yeast. To paraphrase a remark by Charles Darwin, many 

 later embryologists have been "mere schoolboys to old Aristotle." 



Aristotle's work influenced embryological thought for at least twenty 

 centuries, and, with the passage of time, the study of embryology had its 

 ups and downs. During the middle ages an enormous amount of specula- 

 tion regarding development teetered, often precariously, on an exceed- 

 ingly meagre amount of observation. At times embryology and theology 

 joined hands, and we find in embryological endeavors search for a nobler 

 concept of life. Embryology and art were sometimes associated, as in the 

 case of Leonardo da Vinci, who not only left in interesting drawings evi- 

 dence of careful dissections of the pregnant uterus, but also records of 

 quantitative studies on growth. On the basis of surprisingly accurate meas- 

 urements he compared embryonic growth with post-natal growth, in- 

 cluding a study of relative sizes of organs during development. 



Passing rapidly over the years, let us pause for a few moments in the 

 seventeenth century, a period to which modern embryology is particularly 

 in debt. Then lived and worked, to mention a few names only, Leeuwen- 

 hoek, inventor of the microscope; Malpighi, with notions of preformation 

 within egg and embryo; Sir Thomas Browne, of singularly enquiring 

 mind; Walter Needham, physician and experimenter; deGraaf, whose 

 name is perpetuated in the Graafian folHcle of the mammahan ovary; and 

 William Harvey, keen observer and lucid writer. 



William Harvey's book, Exercitationes de Generatio7ie A?ii?nalkmj, was 

 published in 165 1. It must have been in the nature of what would now be 

 called a scientific "best seller." After publication of the first edition in 

 London in 1651, three editions, all in Latin, came out during the same year 

 bearing the imprint of Amsterdam publishers. The first English transla- 

 tion appeared in 1653. The frontispiece of Harvey's book is an engraving 

 of Jove, holding a sphere which represents an egg. From the sphere are 

 being liberated all the animals under heaven, and on it is written, "Ex ovo 

 omnia." Although the epigram does not appear in such form in the text, 

 this dictum, all life from eggs, is the continuing thesis of Harvey's book 

 and was one of his great contributions to embryological knowledge. 



A man of the abihty and versatility of William Harvey, cannot be 

 passed by hastily. As an observer and experimenter of the first order, he 

 wrote, ". . . there be one onely roade to Science, namely, that by which 



1 From the translation of Aristotle's De Generatione Animalium by Arthur Piatt, 

 Oxford Press, 1910. 



