EMBRYOLOGY 1 7 5 



of their work. In fact, nearly all of the experimental work, so called, in 

 embryology remained still on the biological level. It made known many 

 conditions in the development of the egg that had never before been sus- 

 pected, but the appeal to physics and chemistry of the so-called develop- 

 mental mechanics was more often by analogy than by demonstration, and 

 even "chemical embryology" has been largely a description of the kind 

 of chemical compounds found in the tgg and embryo. It is true that the 

 transformation of some of these compounds into the other substances or 

 into the finished product is an essential part of the embryological problem, 

 but the embryologist is very largely concerned with the kinds of reac- 

 tions that lead to the particular changes in form of the embryo, as well 

 as with the origin of substances from other materials. 



The extraordinary fact that an egg with little visible organization de- 

 velops into a complicated adult, with a vast amount of organization, had 

 aroused the interest of the philosophers from Aristotle to Whitehead, and 

 in a broad way they realized the mystery of something happening that 

 had no parallel in other fields of scientific interest. These thinkers were 

 mainly impressed with the kind of organization expressed in form as the 

 most important feature of development, and today this still remains as 

 the most outstanding feature of development. That these changes in form 

 might depend on chemical changes in the embryo was either taken for 

 granted or ignored. 



The most discussed "principle" of philosophy goes under the name of 

 entelechy. The entelechy, supposedly the same idea under that name in 

 Aristotle's teachings, was postulated as a principle, guiding the develop- 

 ment toward a directed end — something beyond and independent of the 

 chemical and physical properties of the materials of the egg; something 

 that without affecting the energy changes directed or regulated such 

 changes, much as human intelligence might control the running or con- 

 struction of a machine. The acceptance of such a principle would seem to 

 make it hardly worth while to use the experimental method to study de- 

 velopment, since it would be directed and regulated by the entelechy. In 

 fact, the more recent doctrine of "the organism as a whole" is not very 

 different from the doctrine of entelechy, except in so far as other ways, 

 by which the whole might be coordinated in an ultra or supermaterialistic 

 way, might be imagined. 



Therefore, unless it be granted that the principles involved in develop- 

 ment are of a different order from physical principles in the broadest and 

 most recent usage of this term, it would seem better to table these meta- 

 physical questions, and to try to discover, despite the amount of time 

 and labor involved, how far a knowledge of the chemical and physical 

 changes taking place in the egg will carry us toward an understanding of 

 the developmental processes. It may, of course, be found that an under- 

 standing of the kind of system present in the egg, sometimes still called the 



