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 a 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 



