98 



Cellular Structure and Activity 



boundaries between chromosomes or chro- 

 monemata are visible. In such nuclei only 

 nucleoli and sometimes heterochromatin are 

 visible (Ris and Mirsky, '49). This compact 

 nucleus is found in many animal and plant 

 tissues. The granular fixation image is due to 

 the shrinking of the chromosome gel in most 

 fixatives. 



In some cells the chromonemata appear to 

 be less closely packed and may become visi- 

 ble in the plaase microscope (tissue culture 

 of mouse kidney, spleen, heart: Fell and 

 Hughes, '49). Such nuclei may contain in 

 addition to the extended chromosomes some 

 other material (karyolymph or nuclear sap). 

 The extreme development in this direction 

 is found in the germinal vesicle of many 

 eggs (for instance, Amphibia) where the 

 chromosomes finally make up only a small 

 fraction of the nuclear volume. The con- 

 tent of such nuclei may have a viscosity 

 little different from that of water (Gray, 

 '27a). 



At present the following points appear to 

 be established: (1) Chromosomes persist as 

 individuals in the resting nucleus. (2) They 

 have a relatively loose structure: except in 

 the heterochromatin the chromonemata are 

 uncoiled and chromatids and half-chromatids 

 are often less closely appressed (see Mar- 

 quardt, '41). (3) The chromosome material 

 (nucleoprotein gel) is in an extended state 

 and in many nuclei fills the entire nuclear 

 space outside of the nucleoli. (4) Extra-chro- 

 mosomal material (nuclear sap) may be 

 present in the interstices of the swollen 

 chromosome gel, where the chromosomes fill 

 the entire nucleus. In other cells it may 

 separate the chromosomes that now come 

 visible in the living cell (phase microscope) 

 or in rare cases it may increase to make up 

 the bulk of the nucleus (germinal vesicle). 



THE NUCLEOLUS 



The nucleolus is an organelle of the rest- 

 ing cell. Chemical and morphological 

 changes indicate that it is involved in cell 

 metabolism but nothing definite is known 

 about its function. During karyokinesis the 

 nucleolus degenerates and is re-formed at 

 telophase in association with a definite re- 

 gion on one or several chromosomes ("nu- 

 cleolar organizer"). Exchange of material 

 between chromosomes and nucleolus has been 

 suggested, but there is no direct evidence for 

 it. The time of dissokition varies from mid- 

 prophase to anaphase or later. If it gets into 

 the spindle it may become divided or moved 



to one or the other side and finally into the 

 cytoplasm. In rapidly dividing cells, for in- 

 stance in early cleavage, no nucleolus is gen- 

 erally formed in the interphase nucleus. 



CHROMOSOME MOVEMENTS 



Following chromosome reproduction and 

 chromosome splitting normal karyokinesis 

 involves a series of chromosome movements 

 in the course of which the chromosome 

 halves are separated into the daughter nu- 

 clei. 



Movements inside the Nuclear Membrane. 

 During interphase little movement of the 

 chromosomes occurs. Often they appear in 

 prophase in typical telophase orientation, 

 with the kinetochores of all chromosomes 

 close together (Rabl orientation). In pro- 

 phase, as they become more compact and 

 the amount of extra chromosomal material 

 increases they begin to show some changes 

 in position. In late prophase they often be- 

 come evenly spaced, preferably on the nu- 

 clear membrane. Where the chromosomes 

 are small and compact this spacing is espe- 

 cially clear. We therefore find the best ex- 

 amples in late meiotic prophase. 



Another type of movement of chromosomes 

 within the nuclear membrane appears to be 

 due to a mysterious relationship between 

 chromosomes (especially chromosome ends) 

 and the centrosome. Some beavitiful examples 

 are foimd in spermatocytes of many insects. 

 In early prophase (leptotene) the ends of 

 all chromosomes come together at one spot 

 opposite the centrosome to form the so-called 

 bouquet stage (cf. Schrader, '53). Later in 

 prophase as the daughter centrosomes move 

 apart, the chromosome ends (or, in case of 

 small chromosomes, entire chromosomes) fol- 

 low along inside the nuclear membrane. The 

 chromosomes are thus separated into two 

 random groups near the centrosome (cf. 

 Hughes-Schrader, '43a). The same occurs in 

 some somatic cells (whitefish cleavage. Fig. 

 12; mouse spleen in tissue culture: Fell and 

 Hughes, '49, Fig. 16). 



Movements on the Spindle. After the nuclear 

 membrane breaks down the spindle takes 

 shape in the former nuclear area and the 

 further movements of chromosomes are in 

 relation to this cell structure. First the 

 chromosomes are moved into the equatorial 

 plane of the spindle where they become 

 spaced in a regular fashion (metakinesis), 

 then after a certain length of time their 

 halves are moved apart toward opposite 

 spindle poles (anaphase movement). 



