THK CKLL NrcLEUS. 151 



muleolus is suspended bv the threads of kiirvo|ilHKiJi. These 

 two Helj,'!!!!! workers also observed timt, under certiiin circum- 

 st;iiiecs, one or more viicuoles luny appear in most of Hansen's 

 species of Siirr/uironii/rt-fiK^ its also iu .Sr/ihoxdrrfiiinimi/rt^ I'mnhf 

 anil divers beer yejists and pressed yeasts, especially when the 

 cells are tmnsferred into a fresh nutrient solution. They have 

 also been obseivcd by liuscalioiii in the nuch-i of >'. 'futfii/iifuj<, 

 luit are never found in Siin-h. Lwlici'jii and Si-hizugiu-charoinijrtfs 



Ol'tOKjHJI'lHt. 



A few remarks may here be made as to the division of the 

 nucleus. The reports hitherto ma<le on tin- form and stiucture 

 of the nucleus of yeast luive always related to cells not actually 

 enga^'ed in re|)roiluction. When, however, this process sets 

 in. a similar ilivision (re]>roduction) of the hitherto cjuie.scent 

 nucleus also bej^ins. This operation, which has l)een alreadv 

 mentioned in |$ 46 and !5 219, may proceed in two ways: either 

 the comparatively simple process of direct subdivision, known 

 as fragmentation <n- amitosis. or by the indirect division termed 

 segmentation, karyokinesis, .n mitosis. In this latter metlnKl 

 the two daujrhter-nudei are the result of exten.sive alterations 

 and modifications of the nuclear .stiucture. as may be read in 

 detail in the treatise of A. ZlMMEitM.w.v (II.) already mentioned. 

 So far as yaist cells are conceined, a certiiin lack of unanimity 

 on this point prevailetl for some time amon<; the various ob- 

 servers. Thus the nuclear ropnxluction acconij)anvin<r jremma- 

 tion was re<,'ariled as fnignientition liy H. >i(KLLKH (11.) in 

 1892, and this opinion was shared by 1)axge.\kd (II.) a year 

 later, BrscALioxi (I.) taking' the s;ime view in i8()6, and H. 

 Waijeu (I.) in 1S98. On the other hand, Ja.vssexs (I ) in 181,3 

 opined that the nucleus in the building yea,^t cell repro«luces Ity 

 karyokinesi.s. Eipially conti-adictorv are the rejKjrts on nuclftir 

 sulnlivision duriuir the formation of a.«<cospores. Jans.«;ens con- 

 sidi'ie<l it karyokinetic. but Mikllek (IV.) and Hiscalioxi 

 held it to be fnigment^ition. The problem was .solved in i8g8 

 by the observation of Jaxssexs and Lebl,\xc (I.) that the inulei 

 of yeast cells, especially S(irrfi(ir>iiin/r/.i Lwhriijii and Srhizn- 

 mi'i'haronnji'fii orfosfmrits, are usually repitKluced bv karvo- 

 kinesis, but that this prtK-ess may under<;o a more or less 

 extensive sinipliiication and approximation to direct ilivision, 

 more paiticularly when the cells are kept untler comiKuatively 

 less favouiable conditions of exi.stence. Similar interme<liate 

 stajjes between the two extremes, of mittKso on the one han<l and 

 amitose on the other, have .also been recindetl in respect of the 

 cell nuclei of hi«;her plants duiing the past few year.s. 



We are also iiulebted to the two last-named Hel;rian workers 

 tor a series t>f observations on the protrie.ss t>f nuclear .miI>- 

 division. The tirst thinir noticeable in a cell that is about to 

 bud is the dis;ippeanince of the nuclear membrane, and the 



