4 INTRODUCTION 



organism is produced from a fertilized egg. It is the purpose of this 

 book to describe "how" the frog develops from the fertilized frog's 

 egg, as we understand it. The facts of this book are made available 

 through the accumulated studies of many investigators, in this and 

 other countries. 



There are those who are interested in the subject matter of embry- 

 ology solely because they were once embryos themselves, or because 

 they anticipate becoming partners in the further production of em- 

 bryos. The closer we can come to the understanding of the mecha- 

 nism of embryonic development of any single species, the closer will 

 we come to the understanding of the mechanism of life itself. The 

 processes of the embryo are certainly fundamental to all of life. Life 

 exists by virtue of successful embryonic development. There are few 

 things in the living world more absorbing or more challenging to 

 watch than the transformation of a single cell into a complex organ- 

 ism with its many organs of varied functions, all most efficiently in- 

 tegrated. 



Embryology is not a new subject. It has a very rich heritage, based 

 upon the solid foundation of pure descriptive morphology. This is 

 followed by the comparison of the variations in development, leading 

 to the recent trend toward the physical and chemical analyses of 

 the developmental processes. In order that we do not lose sight of 

 this heritage, a brief survey is given in the following pages. 



The Period of Descriptive Embryology 



While organisms must have been reproducing and developing 

 since the beginning of life, knowledge of these processes seems to 

 have begun with Aristotle (384-322 B.C.), who first described the 

 development and reproduction of many kinds of organisms in his "De 

 Generatione Animalium." Since he could not locate the small mam- 

 malian egg, he considered its development to be the most advanced 

 of all animals. Below the mammal he placed the shark, whose young 

 develop within the body of the female but are born alive and often 

 with the yolk sac attached. Next, below the sharks, he placed the 

 reptiles and the birds whose eggs are complete in that they are pro- 

 vided with albumen and a shell. The lowest category of development 

 was that of the amphibia and fish, which had what he termed "in- 

 complete" eggs, referring, no doubt, to the method of cleavage. 

 Aristotle believed that development always proceeds from a simple 



