DERIVED ORGANIZATION 



101 



is usually preceded by activity in the nucleus resulting in a skein- 

 like arrangement of the chromatin (spireme) from which definite 

 chromosomes emerge. This spireme, in Metazoa, is the stage of 

 pairing of homologous chromosomes, i. c, chromosomes representing 

 the same characteristics in the two parents. By such association 



D 



c 



Fig. 55. — Monocystis rostrata; chromosome reduction. ^4, Formation of spindle 

 in pseudo-conjugant; B, C, nuclear plates of progamous divisions, 8 chromosomes; 

 D, anaphase of same; E, anaphase of last progamous division, the number of chromo- 

 somes is here reduced from 8 to 4. (After Mulsow.) 



the chromosomes when fully formed are apparently reduced to the 

 haploid number, but each is double, and the actual reduction occurs 

 in the ensuing divisions. 



In Protozoa the antecedent or prophase stages of the first meiotic 

 division rarely conform to the metazoan scheme, but in most cases 



