166 PROTOZOOLOGY 



but increases in size and the chromatin becomes grouped into small 

 masses (6, c), which become associated into a spiral ribbon (d-g). 

 The latter then breaks up into 12 segments that are arranged paral- 

 lel to the axis of the elongating nucleus (h-i). Each segment con- 

 denses into a chromosome which splits longitudinally into two (k) 

 and the two groups of chromosomes move to opposite poles (l-P). In 

 Zelleriella elliptica (Fig. 295) and four other species of the genus in- 

 habiting the colon of Bufo valliceps, Chen (1936, 1948) observed the 

 formation of 24 chromosomes, each of which is connected with a 

 fiber of the intranuclear spindle and splits lengthwise in the meta- 

 phase. 



While in the majority of protozoan mitosis, the chromosomes split 

 longitudinally, there are observations which suggest a transverse di- 

 vision. As examples may be mentioned the chromosomal divisions in 

 Astasia laevis (Belaf), Entosiphon sulcatum (Lackey), and a number 

 of ciliates. In a small number of species observations vary within a 

 species, as, for example, in Peranema trichophorum in which the 

 chromosomes were observed to divide transversely (Hartmann and 

 Chagas) as well as longitudinally (Hall and Powell; Brown). It is 

 inconceivable that the division of the chromosome in a single species 

 of organism is haphazard. The apparent transverse division might be 

 explained by assuming, as Hall (1937) showed in Euglena gracilis, 

 that the splitting is not completed at once and the pulling force act- 

 ing upon them soon after division, brings forth the long chromo- 

 somes still connected at one end. Thus the chromosomes remain to- 

 gether before the anaphase begins. 



In the instances considered on the preceding pages, the so-called 

 chromosomes found in them, appear to be essentially similar in 

 structure and behavior to typical metazoan chromosomes. In many 

 other cases, the so-called chromosomes or "pseudochromosomes" 

 are slightly enlarged chromatin granules which differ from the ordin- 

 ary chromatin granules in their time of appearance and movement 

 only. In these cases it is of course not possible at present to deter- 

 mine how and when their division occurs before separating to the 

 respective division pole. In Table 5 are listed the number of the 

 "chromosomes" which have been reported by various investigators 

 in the Protozoa that are mentioned in the present work. 



Cytoplasmic division 



The division of the nucleus is accompanied by division of extranu- 

 clear organelles such as chromatophores, pyrenoids, etc. The blepha- 

 roplast of the flagellates and kinetosomes of the ciliates undergo di- 



