GERM-CELL CYCLE IN ANIMALS 



CHAPTER I 



INTRODUCTION 



Since the enunciation by Harvey of the aphorism 

 Omne vivum ex ovo in the seventeenth century, the 

 statement has frequently been made that every 

 animal begins its individual existence as an egg. 

 While this is not strictly true, since no eggs occur in 

 the life history of many one-celled animals (Pro- 

 tozoa), and a large number of multicellular animals 

 (Metazoa) are known to develop from buds or by 

 fission, still the majority of animals arise from a single 

 cell — the egg (Fig. 4, A) . In most cases this egg, 

 or female sex-cell, is unable to develop in nature 

 unless it is penetrated by a spermatozoon or male 

 sex-cell (Fig. 4, B) . The single cell resulting from the 

 fusion of an egg and a spermatozoon is known as a 

 zygote. One of the most remarkable of all phenom- 

 ena is the development of a large, complex organism 

 from a minute, and apparently simple, zygote. 



According to the older scientists, a miniature of the 

 adult individual was present in the egg, and devel- 

 opment consisted in the growth and expansion of 

 B 1 



