A critical study of the cytology of Crepis virens. 
131 
The age of the tissue may also have sorae bearing on the character 
of the chromatic contents of its resting nuclei. Rosenberg (45) found 
that “Chromatinklümpchen” are more constant in nuniber in nuclei of 
tissues that have completed their growth (p. 402). 
The facts enunierated above show that the same series of nuclei may 
exhibit different degrees of chromatic concentration ; that this concentra- 
tion may be altered by the specific fixmg reagent used, and by the strength 
of the Solution, and that physiological conditions such as nutrition and age 
affect the aniount and disposition of the chromatic contents. When it 
is reahsed that all these factors have to be taken into consideration it 
is not to be wondered at that the results of observations on resting 
nuclei are often not iu accordance. 
Another, though minor, explanation for the diversity in pubüshed 
records is the difficulty of obtaining accurate counts of chromatic aggre- 
gations. Even Overton, who introduced the term ‘prochromosome’ into 
literature, and definitely correlated the numbers of the ‘prochromosomes’ 
with those of the chromosomes, was mistaken in his calculations. In his 
first paper on Thalictrum (40) he States that 24 is the somatic number of 
chromosomes, and that 24 prochromosomes appear in the presynaptic 
phases and pair as they go into synapsis. Subsequently (41) he realised 
that there were in fact 48 prochromosomes, the 24 representing pairs of 
prochromosomes. The oversight was due to the dose association of the 
individuals of each pair. Moreover it is often difficult to discrimüiate 
between typical chromatic aggregations and other bodies, possibly chro- 
matic granules and nucleolar fragments, which are constantly present in 
resting nuclei, and which afford controversial arguments as to their identity. 
Davis (6 and 7) has emphasized the impossibility of arriving at an ac- 
curate determination of the numbers. On the other hand, Rosenberg (48) 
foimd httle difficulty in counting the prochromosomes of resting nuclei 
as he included all bodies that took the Haematoxylin stain (p. 51). 
Those who regard chromatic bodies as ‘prochromosomes’ believe their 
appearance in the same number of the chromosomes to be indicative 
of their true nature. On the other hand, those who consider chi’omatic 
bodies to be merely aggregations of chromatin, in view of their number 
being variable, regard the fact that the number of chromatic bodies present 
in the resting nucleus is sometimes identical with that of the chromosomes, 
as a matter of chance to which no imdue importance should be attached. 
The unstable nature of the chromatic bodies has already been dis- 
cussed, and it seems to be hardly possible to regard such variable struc- 
tures as ‘prochromosomes’. It now remains to enumerate two definite 
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