Notes on Recent Literature. i 51 
great difficulty of coming to an exact determination upon this point 
is emphasized by the fact that in his last paper, Overton gives the 
number of chromosomes in Thalictrnni pnrpurascens as forty-eight 
—twice as many as the number indicated by Strasburger, 1 and by 
Overton himself in his earlier paper when he “ mistook the pairs of 
chromosomes for a single chromosome.” 
For a detailed discussion on the permanence of chromosomes, 
the double nature of the somatic nucleus, and the conjugation 
between pairs of chromosomes in the prophase of the heterotype 
division, the reader must be referred to Overton’s paper. Cytologists 
will be grateful for the excellent summary of the literature upon 
these subjects which is also given. 
One feels that in the study of forms such as those described by 
Overton, where the chromosomes can be traced individually 
throughout all conditions of the nucleus, lies the most promise for 
the elucidation of the manner in which the pairing of the chromo¬ 
somes takes place. One of the main objects of work of this kind, 
as is very clearly shown by the general trend of the discussions 
given by each author, is the attempt to find a satisfactory cytological 
basis for the segregation of characters which is known to take place 
in (or prior to) the formation of the germ-cells in many plants and 
animals. The pairing of homologous chromosomes, and their 
separation in the heterotype division, affords the promise of a solution, 
but while we find two theories arriving at this desirable end by such 
dissimilar routes, it is not altogether surprising that the sceptic 
persists in his unbelief. 
Indeed, in the absence of evidence more definitely connecting 
the chromatin with the hypothetical material particles (pangens) 
whose existence has been postulated in order to explain the 
transmission of inherited characters, and with the exception of the 
remarkable facts brought to light by the observations of Wilson 
and others upon the determination of sex in the Hemiptera, etc., 
almost the strongest presumption in favour of the significance 
usually attributed to the heterotype division, lies in the change of 
symmetry which takes place in the divisions of the nuclei at that 
point. In the somatic cells there exists one form of symmetry, in 
that each of the chromosomes undergoes a longitudinal fission, and 
the daughter nuclei are, therefore, so far as our knowledge goes, 
qualitatively exactly similar to one another and to the parent nucleus 
from which they originated. In the heterotype division another 
type of symmetry supervenes, in that the chromosomes become 
associated in pairs and each daughter nucleus receives one member 
from each pair. Each daughter nucleus therefore receives a single 
series of chromosomes which are individually and collectively 
homologous, but not necessarily identical in quality, with those of 
the sister nucleus. 
This change of symmetry corresponds exactly with that which 
the results of experimental work demand. The facts of regeneration 
show that, in general, the various regions of the soma of a plant 
retain the potentiality for the development of all the characters 
proper to the plant; translated into terms of material particles or 
pangens, this means that the pangens have been distributed 
Sitzber. Akad. Wiss. Berlin, XVIII., 1904, p. 604. 
i 
