DERIVED ORGANIZATION 



103 



of the chromosomes, but in the coccidian Aggregate, eberthi where 

 reduction is zygotic (the vegetative stages being haploid) the twelve 

 chromosomes unite in six pairs of homologous chromosomes (Dobell) 

 (Fig. 56) and a modified spireme occurs in the progamous divisions. 

 Similar but less definite conditions are shown in the gregarine 

 Diplocystis schieideri as described by Jameson (1920) (Fig. 158, 

 p. 310). A somewhat simplified history of the chromatin was 

 given by Mulsow (1911) for the progamete nucleus of the gregarine 

 Monocystis rostrata (Fig. 55). Here, differing from Diplocystis, re- 

 duction is gametic and the vegetative stages are diploid. The 

 resting nucleus is vesicular and the chromatin granules join chain- 

 wise to form eight chromosomes. These split lengthwise in the 

 metaphase stage, a preliminary spireme stage, apparently, being 

 absent. 



Fig. 57. — Micronucleus of Paramecium caudatum in the prophases of the first 

 meiotic division. .4, Early stage in the formation of chromosomes; B, elongation 

 of the nucleus prior to crescent formation ; C, metaphase of the first division. Dehorne 

 describes the entire chromatin aggregate as forming one highly convoluted chromo- 

 some. (After Dehorne.) 



In the hypermastigida (Trichonympha, Dinenympha, Stauro- 

 joenina, etc.) flagellates, fertilization is unknown, but ordinary 

 nuclear division is preceded by formation of long chromosomes 

 which give the appearance of a spireme. 



Quite a divergent type of spireme formation is found in the 

 ciliates where the chromatin is massed in homogeneous micronuclei. 

 In Paramecium caudatum the micronucleus elongates to form a bar 

 nearly equal in length to the macro nucleus (Fig. 57). The massed 

 chromatin becomes granular, and the granules stretch out in an 

 elongate network which, in the following crescent phase, breaks up 

 into a multitude of double chromosomes. 



