178 READINGS IN BIOLOGICAL SCIENCE 



the substances manufactured by the genes. The initial differences in the 

 protoplasmic regions may be supposed to affect the activity of the genes. 

 The genes will then in turn affect the protoplasm, which will start a new 

 series of reciprocal reactions. In this way we can picture to ourselves the 

 gradual elaboration and differentiation of the various regions of the em- 

 bryo. 



-W\ ^ y y 



OLD PROBLEMS AND NEW IN 

 EXPERIMENTAL EMBRYOLOGY * 



E. G. BUTLER 



Ever since man possessed the ability for correlated mental activity he 

 has undoubtedly speculated on the phenomenon of embryonic develop- 

 ment. The manner in which a living organism comes into being, takes on 

 definite organized form, undergoes the establishment of vital functions, 

 and, finally, assumes the characteristics of a complex adult body, seldom 

 fails to elicit the interest of intelligent human beings. 



Although the study of embryology is rooted deep in antiquity, refer- 

 ences in pre-Grecian literature to developing organisms are, unfortunately, 

 somewhat fragmentary. During the rise of Greek culture, however, nearly 

 all philosophers devoted some attention to problems of development and 

 several wrote extensively on the subject. With Hippocrates, observation 

 began to take precedence over speculation, and he and his associates left 

 records indicating considerable time devoted to the study of embryos in 

 a variety of animals. Finally, toward the end of the fourth century, b. c, 

 Aristotle wrote the first book dealing solely with embryonic development, 

 "De Generatione Animalium." 



The son of a physician, Aristotle was reared on the works of the Hip- 

 pocratic school and was early trained in methods of dissection. It is doubt- 

 ful if any of Aristotle's scientific work excelled that which he did on 

 embryology. His methods and tools were inadequate, his observations 

 often too few, and many of his conclusions far from correct, nevertheless, 

 he arrived at an insight into animal development far exceeding that of any 

 predecessor. He seems to have appreciated that embryonic development 

 is not a mere unfolding and growth of structures already established, and 

 he resorted, not infrequently, to physical or mechanical comparisons. 

 Referring to the developing parts of an embryo, he wrote, "It is possible, 

 then, that A should move B, and B move C; that, in fact, the case should 

 be the same as with automatic machines shown as curiosities. For the parts 



* Based upon an address presented at the annual dinner meeting of the Princeton 

 Chapter, of Sigma Xi, May 191 1, and adapted here by permission of the author and 

 of the Society of the Sigma Xi. 



