I MITOSIS 9 



condensation ultimately form the relatively short and thick chromosomes 

 of the later stages. Karyosomes, if present, since they are composed of 

 chromatin, disappear, being used up with the rest of the chromatin in 

 the formation of the chromosomes. If plasmosomes are present, they dis- 

 appear either before or after the disappearance of the nuclear membrane 

 (see below), apparently without participating in the formation of the 

 chromosomes or playing any further part in the hfe-history of the nucleus. 

 For some time after the thread formation, which starts in the early 

 prophase, has proceeded or even been completed (by conversion of the 

 entire chromatin content of the nucleus into filaments), the length of the 

 threads is far greater than the circumference of the nucleus (Figs. 3, 6, 

 7, 8), and hence the nucleus is filled with a complicated tangle— the 

 spireme of Flemming — in which it is impossible to discern how many 

 separate filaments are present. There may indeed be no apparent 

 breaks in the thread, and when such do appear it is often difficult to 

 determine whether they are real interruptions of continuity, or merely 

 the optical effects of a sharp angle 

 in the thread, etc. This gave rise 

 to the old view that in the earlv 

 prophase there is but a single 

 greatly convoluted thread present, 

 and so this stage was known as 

 the continuous or unsegmented Fig. 5 



spireme, in COntra-distinCtion to Meiotic prophase {i)\n Oenothera rubrinervis showing 



, cohering chromosomes. (Gates, Botanical Gazette, 1908.) 



the segmented spireme of the A,uncut nucleus showing single thick spireme ; B, later 



stage showing the spireme segmenting into chromosomes. 



later prophase, m which a num- 

 ber of separate threads is plainly present. ' However, careful examina- 

 tion, and especially comparison with forms in which the nuclei are poorer 

 in chromatin and the prophase filaments consequently less voluminous, 

 has led very generally to the conclusion that the conception of the con- 

 tinuous spireme as a constant stage in prophase is incorrect, but that at 

 all stages of the prophase the spireme generally consists of as many 

 separate segments as there will ultimately be chromosomes. In fact, the 

 spireme is a tangle of very long thin chromosomes. It is not, however, 

 uncommon for even fully formed chromosomes to cohere by their ends 

 (Fig. 5), and there is no doubt that this occasionally happens in tin- 

 case of the early prophase filaments also ; in this way a continuous 

 spireme might be formed, but its continuity would be, so to speak, 

 accidental and not an essential feature of it. 



A striking and important characteristic of the prophase threads or 

 chromosomes in many species is their duplicity. This can often be 

 observed from the very beginning of the prophase (Figs. 3, 8), and is 

 caused by the longitudinal division of each chromosome into two 



A ^ ' B 



