324 BOTANICAL GAZETTE [May 
mation of two somatic chromosomes which may or may not show 
fission. The attempt to interpret bivalent structures as “tetrads”’ 
has been a frequent source of error. The significance of the term 
“tetrad” as applying to the heterotypic chromosomes was of 
course derived from the supposed theoretical implications, derived 
from WEISMANN’s distinction between a longitudinal or quanti- 
tative, and a transverse or qualitative division of the chromosomes. 
It was assumed that if the heterotypic chromosomes gave the 
appearance of being tetrads or tetravalent structures, then they 
must have originated from two segmentations, one of which was 
longitudinal of the spirem and the other transverse. This con- 
ception has since lost much of its usefulness, and this is a case 
where the too close adherence to a useful and stimulating theory 
has tended finally to retard progress. While such a difference 1s 
not impossible, it has not yet been shown by critical observation 
or experiment that there is any fundamental distinction between 
a longitudinal and a transverse segmentation of a chromosome. 
But, on the other hand, many facts with regard to chromosome 
continuity or individuality of a certain type must be regarded as 
well established. Longitudinal fission of all the chromosomes 18 
regarded as universal in somatic mitoses, and this has frequently 
been pointed to as a strong argument in favor of the view that a 
chromosome is composed of qualitatively different portions oF 
bodies arranged along its long diameter, whose equal division an 
distribution it is the function of this longitudinal fission to bring 
about. But it may be pointed out that this longitudinal fission 
of the viscous chromosome bodies may be determined by purely 
physical forces resulting, for example, from the electrical charges 
carried by the chromatin particles. 
If this were the case, instead of an equal distribution of “ids. 
to each daughter chromosome, longitudinal fission would mean 
merely fission for mechanical or physical reasons along whatever 
becomes the longitudinal axis of the chromosome as it changes 
from the reticulate condition of the resting nucleus to the compact 
condition of prophase or metaphase. ‘There is no observational 
evidence that longitudinal fission means any more than this, nOF 
that in the passage from the alveolate or reticulate condition of the 
