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- 
tTozoa), and a large number of multicellular animals 
(Merazoa) 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 spermatozoén or male 
sex-cell (Fig. 4, B). The single cell resulting from the 
fusion of an egg and a spermatozoién 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 
