( YTOI.OCiY 



fluoiu-fd liy tlio ('••irlior di-sc-riiitions iif Niijilcr. C'li;it- 

 toii, Ah'xicff. I'tc cif promitosis in .Initichn. IIow- 

 i-vt-r. oiu- in.-irkrd (lifftTfiU'C i.s iipiiart-nt. In tiic I'las- 

 niodiopliorali's tlii' kjirvosonu' (lot's not apiu-.ir to 

 fiiiu'tion as a nucli-o-i-ontrosomc durinjc division. In 

 sevt-ral jriiura of this ordiT cU-arly-detint'd crntro 

 sonu's and astral rays have Ucvn rci)ortfd. strui'turts 

 wliicli arc latkina- i!i tlic mitoses shown in fiiiurts 2 !■ 

 to ."i.'t. \\ Inthir or not the division spindle orii;inates 

 in relation to the eentrosonies in the i'lasnio(iio|)ho- 

 rales is still uneertain. The most strikinj; ditlerenee. 

 however, between the two types of division in these 

 two a:rou|)s. aeeordins; to Terhy. Home, and Webb, 

 is the jiresenee of sharply-defined ehromosomes in 

 the i)rophases and the equatorial ring stages. This 

 differenee will beeome more a])i).irent in the discus- 

 sion which follows. 



Turnini; now to the observati(nis of Terhy, Home, 

 and Webb, these workers contended tii.it the failure 

 of previous investigators to find ciiromosomes in the 

 vegetative divisions was due to insufficient study of 

 the propliases where the chromosomes originate. In 

 Spotuiospora and Sorospharra, Home found wheel- 

 like resting nuclei (fig. 3t) with large karyosomes. 

 radially oriented achromatic strands, and numerous 

 chromatin granules, but he did not regard this as a 

 constant and static structure of resting nuclei. Ac- 

 cording to him. the structure may change during the 

 various developmental phases. In Sorosphaera he 

 found consiiieuous centrosomes and astral rays dur- 

 ing the i5roi)hases (tig. 3.5). but such structures were 

 never observed by U'cbb. possibly because the latter 

 eniploved a strictly nuclear stain. By using a modifi- 

 cation of Newt(Mi's gentian violet-iodine method, 

 Webb was able to detect small chromatin granules 

 connected by fine threads throughout the resting nu- 

 cleus. The first visible evidence of division is an in- 

 crease in the staining capacity of the chromatin gran- 

 ules (fig. 36) which soon becomes alined on the 

 threads (fig. 37). according to Webb. These threads 

 contract as they move away from the ))eri))hery and 

 form slender chromosomes (fig. 1-0) directly, with- 

 out ))revious deveIoi)ment of a coiled spir<nie stage. 

 Home likewise observed a thickening of chromatic 

 rods projecting from the nuclcolc as the first indica- 

 tion of |)ro))liase. Later an irregular chromatic net- 

 work emerges which goes into a tyi)ieal coiled spi- 

 reme (fig. 39) from which the chromosomes even- 

 tu.allv emerge. Hornc found mimerous ))ost-s))iremc 

 configurations with only two or three \'-sha))ed chro- 

 mosome, but he nevertheless believed that the ha))- 

 loid number in Spone/ospora is four. Miss Terby 

 also found mimerous rods and threads in the pro- 

 phase nuclei of P. Brasxicae (fig. 38) from which 

 the chromosomes are subsequently formed. Milo- 

 vidov. on the other hand, was unabh' to recognize 

 chromosomes in material stained Viy l-'eulgen method. 

 Returning to Webb's account of Sorosphaera, the 

 four chromosomes contract further in the prophases 

 and become V- and U-.shaped (fig. H. Vh). and soon 

 thereafter s)>lit ends become visible (fig. t2). indi- 

 cating a splitting of the chromosomes in jirejiaration 



for division. Uj) to this time the nueleole reni.iiiis 

 more or less globui.ir. but it soon begins to elongate 

 in the direction of the poles. The chromosomes then 

 become arranged end to end in .-in irregular, liroken 

 ring in the equator of the nucleus (fig. 13). \ jiol.ir 

 view of such a stage is shown in figure H- with the 

 s))lit .and twisted chromosomes lying near the i)cripli- 

 ery of the nuclear menibrane. Following this stage, 

 ihev contract and thicken, so th.-it the longitudinal 

 split is no longer \ isible (tig. Ki). The ehroniosomes. 

 nonetheless, retain their iiuiividu.ility . according to 

 Home's and Webb's dr.-iwings. .'is is shown by the 

 breaks in the equatorial ring (fig. K>. IS, li)). This 

 ring stage persists for a comparatively long time 

 and is the one most frequently observed in the vege- 

 tative divisions. 



,\eeording to Home's and Webb's figures, the 

 elongate nueleole may become slightly constricted at 

 this stage in preparation for division (tig. Vd. IS). 

 .Miss Terby ('"^3). however, found that the nucleolar 

 changes vary considerably in P. Brassicar. Instead 

 of constricting and dividing more or less equally, it 

 may fragment into two or more unequal parts (fig. 

 Vl) or move intact as a single body to one of the 

 poles. Oftentimes, jiarts of it remain stranded be- 

 tween the daughter nuclei (fig. GK 65) as in higher 

 plants. 



The metaphase split reapjiears first in the median 

 region of the chromosomes (fig. 51 ) at the conclusion 

 of the equatorial ring stage and travels outward to 

 the ends, which suggested to Webb that the spin- 

 dle fiber attachment is median. As the chromosome 

 halves separate, two daughter rings are formed (fig. 

 52) which migrate toward the opposite poles (fig. 

 33-57) until they reach the ends of the elongate 

 nueleole (fig. 56, 58. 59). According to Webb, the 

 nueleole in Sorosphaera does not constrict as a rule 

 until telophase (fig. 57-59). The two parts finally 

 separate and become surrounded by daughter ehro- 

 mosomes (fig. 60) at the poles of the nucleus. The 

 nuclear membrane then constricts and divides in 

 much the same manner as the nueleole .and thus forms 

 the daughter nuclei (fig. 61). The ciiromosomes ad- 

 iiere to the nueleole at first, but later sejiarate from 

 it. Miss Terby. however, maintained that the nucleoli 

 are formed anew at each telophase in P. Brassicae. 

 .\fter the daughter nuclei have been formed, the 

 chromatin mass gives ofl' material which unites to 

 form the daughter nucleoli (fig. 62. 63). .As to the 

 origin of the daughter nuclear areas. Miss Terby 

 ('23) reported that they begin in the projihases as 

 two hyaline vesicles on the jiolar sides of tiie nu- 

 eleole. As the latter elongates, divides, and the two 

 segments separate, the vesicles ])recede them to the 

 |)olcs of the nucleus. The vesicles then pass through 

 the nuclear membrane at the ))oles and expand, and 

 shortly thereafter the d.aughter nuclcol.ar segments 

 and chromatin enter .uid are thus incor|)or;ited in the 

 vesicles. The boundaries of the vesicles become the 

 nuclear membranes and thus constitute the limits 

 of the daughter nuclei. In a later paper, however. 

 Miss Terby ('32j modified this account and rejiorted 



