CHAPTER ONE 

 Introduction 



The Period of Descriptive Embryology Epigenesis vs. Preformationism 



The Period of Comparative Embryology Soma and Germ Plasm 



The Period of Cellular Embryology The Germ Layer Concept 



The Period of Experimental Embryology The Normal Sequence of Events in 



The Embryologist as a Scientist Embryology 



Why the Embryology of the Frog? Cell Multiplication 



Some General Concepts in Embryology Cell Differentiation (Specialization) 



Biogenesis Organogeny 



Biogenetic Law — The Law of Re- Growth 

 capitulation 



Embryology is a study of early development from the fertilized egg 

 to tlie appearance of a definitive organism. The stage is generally 

 referred to as its embryonic stage. It is not so inclusive a term as 

 ontogeny, which refers to the entire life history of an organism from 

 the fertilized egg to old age and death. Since either the sperm or the 

 egg (of a zygote) can affect development, the study of embryology 

 might well begin with a study of the normal production and matura- 

 tion of the germ cells (gametes). It should include fertilization, cleav- 

 age, blastula and gastrula formation, histogenesis, organogenesis, and 

 the nervous and humoral integrations of these newly developed organs 

 into a harmoniously functioning organism. When development has 

 proceeded to the stage where the embryo can be recognized as an 

 organism, structurally similar to its parents, it no longer can be re- 

 garded as an embryo. 



The unfertilized egg is an organism with unexpressed potentialities 

 derived from an ovary containing many essentially similar eggs. Its 

 basic pattern of development is maternally derived and is predeter- 

 mined in the ovary but its genetic complex is determined at the very 

 instant of fertilization. This predetermination is in no sense struc- 

 tural (i.e., one never has seen a tadpole in a frog's egg) but is none- 

 theless fixed by both the nuclear and the cytoplasmic influences of 

 the fertilized egg (or zygote). These influences are probably both 

 physical and chemical in nature. 



No amount of environmental change can so alter the development 



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