l80 READINGS IN BIOLOGICAL SCIENCE 



we proceed from things more known, to things known less; and from that 

 which is more manifest, to that which is more obscure; . . . ." ^ Prob- 

 lems of growth and differentiation puzzled Harvey, and will puzzle us 

 in some detail in a few moments. However, he handled these problems 

 generally in a manner much better than his predecessors or contemporaries. 

 Some have held that he sought to endow developing embryos with an 

 imminent spirit of special sort, but as W, K. Brooks pointed out years ago, 

 when Harvey referred to a "vital principle," he probably meant to say 

 that the embryo was ahve, with no thought of implying supernatural 

 agencies. Parts of embryos, Harvey wrote, "are at once similar and dis- 

 similar, and from a small similar is a great organ made." ^ This might be 

 taken as a text by modern students of embryology, who discuss embryonic 

 localization and determination, and construct for us maps of the egg to 

 show the presumptive fate of each part. 



Embryonic development in higher animals involves an orderly sequence 

 of events, including production of sex cells, the ovum or egg in the female, 

 the spermatozoon in the male; fertilization, which is the union of an egg 

 and a spermatozoon; and, following fertilization, all of those processes 

 concerned with growth and differentiation of the new individual. As I 

 have mentioned, eggs from various types of animals have long been a 

 subject of study. Spermatozoa were first seen by a man named Dr. Ham 

 and reported to the Royal Society in 1677. Knowing little about the 

 structure of the tgg and the spermatozoon, and less about the nature and 

 significance of fertilization, it is not surprising that many early embry- 

 ologists were led by their inaccurate observations down the easy path to 

 the theory of preformation. It was the simplest way — to make the deduc- 

 tion that all structures of the adult body were present in miniature, in 

 other words, preformed, either in the egg or in the spermatozoon. And, 

 as one would anticipate, two conflicting schools of thought arose. One 

 school, called the ovists, insisting that all adult structures were preformed 

 in the egg; the other school, called the spermists, insisting that all structures 

 were preformed in the spermatozoon. For the adherents to either school, 

 embryonic development meant an unfolding of structures already present. 



1 never tire of reading the description of a spermatozoon written in 1699 

 by one of the zealous spermists, Dalenpatius. It is beautifully phrased 

 and thoroughly inaccurate. Speaking of human spermatozoa he wrote: 

 "They move with wonderful rapidity and by the strokes of their tails 

 produce little waves in the substance in which they swim. But who would 

 believe that in these a human body was hidden? Yet we have seen such 

 with our own eyes. For while we were observing them attentively, a large 

 one threw off its surrounding membrane and appeared naked, showing 



2 From the 1653 translation of Harvey's De Generatione Animaliu7n, printed by 

 James Young for Octavian PuUeyn, London. (Copy in the New York Academy of 

 Medicine.) 



3 Ibid. 



