INTRODUCTION 17 
Metuops oF Repropuction. In the beginning 
paragraph of this chapter it was stated, with reserva- 
tions, that every individual develops from an egg. 
Before we can discuss the germ-cell cycle intelli- 
gently, however, we must consider the exceptions 
to this rule, and outline as briefly as possible the 
various methods of reproduction which are known 
to occur among animals. Reproduction is the forma- 
tion of new individuals by division ; this is frequently 
preceded by conjugation (in the Protozoa) or fertil- 
ization (in both the Protozoa and the MeErazoa). 
Three principal methods of reproduction occur 
in the Protozoa. (1) Binary fission appears to 
be the most primitive. The individual divides into 
two parts which are similar in size and structure; 
these grow into cells like the original parent. Many 
Cruuata, FuaGeiuata, and Rurizopopa normally 
reproduce in this way. (2) Budding occurs when 
a small outgrowth or bud separates from the parent 
cell. This method occurs among the Sucrorta, 
Rapriouaria, Heniozoa, Cimiata, and Myxosro- 
RIpIA. (3) Sporulation results from the division of 
the nucleus of the parent into many daughter nuclei 
and a subsequent division of the cell into as many 
“spores” as there are nuclei. This process is 
characteristic of the Sporozoa and also is found 
among the Ruizopopa. Conjugation is of frequent 
occurrence in the Protozoa. Two or more indi- 
viduals may become connected without fusion of 
nuclei or cytoplasm, thus forming colonies; a pair of 
individuals may unite either temporarily or per- 
Cc 
