No. 463] STUDIES ON PLANT CELL.— VI. 497 
(2) Farmer and Moore, Gregory, Williams, and Strasburger 
hold that there is primarily only a single chromatic thread in the 
nucleus of the spore mother-cell which is the spirem of synapsis 
and the heterotypic mitosis and which most of these authors 
believe to be composed of the full number of chromosomes 
(sporophytic) joined end to end. This spirem splits longitu- 
dinally but the fission is a premature division which prepares the 
chromosomes for the homotypic mitosis. The chromosomes of 
the heterotypic mitosis are formed from loops of the spirem 
which include a pair of sporophytic chromosomes joined end to 
end. The members of this pair come to lie side by side by an 
approximation of the arms of the loops and a breaking apart at 
the head of the structure. This transverse fission of the spirem 
is not. of course a transverse division of a chromosome but 
merely the separation of a pair of chromosomes joined end to 
end. The line between the two arms of the loop marks a region 
of contact due to approximation and not a line of fission. The 
heterotypic mitosis effects a numerical reduction of the chromo- 
somes as in the first view but these chromosomes are formed 
on entirely different principles. A single premature fission of 
the spirem or its segments prepares the chromosomes for the 
' homotypic mitosis. 
Comparing the two schools, it may be noted that they both 
explain reduction phenomena as à numerical reduction of the 
double set of sporophytic chromosomes by a distribution in two 
sets. The fission of the chromosomes is always quantitative and 
there is no hint in any of the views of a qualitative division in 
Weismann's sense. Furthermore, most of the investigators are 
firmly convinced of the individuality of the chromosomes which 
means that they are convinced as morphological entities persist- 
ing from one generation to the next. This is an. important 
agreement in relation to theories of heredity and hybridization 
which we shall discuss at. another time (see treatment of 
“ Hybridization ”). The differences lie in questions of. fact 
regarding the organization of these chromosomes in the spore 
mother-cell and their behavior during synapsis and at other 
periods of prophase in the heterotypic mitosis. There is entire 
accord in that the chromosomes of the homotypic mitosis appear 
