DERIVED ORGANIZATION 99 



sion stages the chromomeres are rearranged in rods or fibrils which 

 form a more or less definite skein within the nucleus; this skein 

 fragments into a large number of chromosomes which, according to 

 Schewiakoff, are longitudinally divided. A more aberrant history 

 is followed by the chromatin of the nuclei of various species of 

 Paramecium. In Paramecium caudatum the micronucleus belongs 

 to the massive type, and there is no satisfactory account of the 

 origin of chromosomes in vegetative division (Fig. 35, p. G7), but 

 the number is much smaller than in the meiotic divisions (see Fig. 

 147, p. 297). 



A more definite metazoan type of chromosome formation is shown 

 by the organisms with a definite number of chromosomes which is 

 reduced to one-half at meiosis. Here the number of. chromosomes 

 is usually smaller and their individual history during nuclear divi- 

 sion is less difficult to make out. A good example, typical of the 

 more complex flagellates, is Trichonympha campanula, as described 

 by Kofoid and Swezy. Here the resting nucleus contains a large 

 granular endosome. In the prophase of division the granules of 

 this endosome give off chromatin along the walls of the linin 

 reticulum until a definite skein stage results (Fig. 54). Double 

 chromosomes, 2(1 in number, and formed by the splitting of the 

 spireme segments, make up a definite nuclear plate. They are 

 attached by intranuclear fibers to the daughter blepharoplasts and 

 are divided longitudinally with the division of the nucleus. The 

 original connecting fibrils between the separating halves of the 

 blepharoplast ('' centroblepharoplast ") remain at all times outside 

 the nuclear membrane, hence it is called a paradesmose by Kofoid 

 and Swezy. One of the chromosomes appears to be different from 

 the others, both in resting and division stages, and is called the 

 heterochromosome, although its function or significance is quite 

 unknown. Similar odd chromosomes are known in some Gregar- 

 inidae and Coccidiida where the vegetative stages are haploid, as 

 well as in other polymastigote flagellates. Except for the complica- 

 tions brought in by the extensive neuromotor apparatus of Trich- 

 onympha campanula, the division figures of other related flagellates 

 are quite similar, although the number of chromosomes is usually 

 smaller. Thus Kofoid and his collaborators found about 24 in 

 Leidyopsis sphaerica, 12 in- Trichomitus termitidis and 4 in Giardia 

 maris (Fig. 54, p. 100). 



A smaller number of chromosomes is likewise found in a number 

 of the Gregarinida, and their history in division approaches that of 

 metazoan chromosomes. Thus in the case of Monocystis rostrata 

 Mulsow describes 8 definite chromosomes formed from a portion of 

 the nuclear chromatin, the number being reduced to 4 in the gamete- 

 forming divisions (Fig. 55). Shellack and Leger, also, have described 

 similar chromosomes in Monocystis ovata and in Stylorhynchus longi- 



