INTRODUCTION 17 



METHODS OF REPRODUCTION. 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 METAZOA). 



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 

 CILIATA, FLAGELLATA, and RHIZOPODA normally 

 reproduce in this way. (2) Budding occurs when 

 a small outgrowth or bud separates from the parent 

 cell. This method occurs among the SUCTORIA, 

 RADIOLARIA, HELIOZOA, CILIATA, and MYXOSPO- 

 RIDIA. (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 RHIZOPODA. 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- 



