26 ANNALS NEW YORK ACADEMY OF SCIENCES 



Sporogenesis is also a critical stage in ontogeny. It is equally critical, 

 or perhaps more critical, than fertilization. 



Irregularities in chromosome behavior are to be found, however, in 

 the cell divisions of somatic tissues. In the root tips of the Europa 

 clon abnormal chromosome numbers readily develop through non-dis- 

 tribution. Furthermore, we find no evidence that somatic nuclei which 

 receive a low number of chromosomes or a number less than 12 can con- 

 tinue in division. For such cells there is cessation of division, which 

 amounts to a tissue abortion. Such irregularities are without a doubt 

 due to the expressions of the same sort of incompatibilities with which 

 we are more familiar in the forms of sterility in reproductive organs. 

 The behavior of chromosomes in the somatic cells of hybrids and of 

 polyploids has been too little studied; it may be much more erratic than 

 is now supposed. 



The irregularities and abortions which appear in mitosis and in 

 meiosis indicate that the destructive interactions develop especially 

 when (a) chromosomes are in relations with kinoplasm, (b) when 

 chromosomes are in processes of splitting or fragmenting and (c) when 

 the activities of pairing and disjunction of chromatin units are in 

 progress. The relations of kinoplasmic activities to chromosomes are 

 important aspects of chromosome behavior in nuclear divisions. In 

 somatic divisions the kinoplasmic activities complete nuclear division 

 where non-distribution is very pronounced (PI. I, Figs. 3, 4 and 5) and 

 this may continue for certain cells that have received increased numbers 

 of chromosomes. In sporogenesis kinoplasmic activities become very ir- 

 regular. With the lagging of chromosomes in the' first meiotic division 

 micronuclei form, and many, or perhaps all of these, do not divide 

 further. Cell plates form after the first division with the greatest of 

 irregularity and, when there is much fragmentation and multiplication 

 of chromosomes, there is delayed action of kinoplasmic structures. 



The meiotic divisions are obviously more complicated than are <i>niati<- 

 divisions both in the activities of chromatin and in the role of kino- 

 plasm. Synapsis and the relations that follow between chromosomes 

 bring chromatin elements into more intimate interactions than previously 

 existed. Disjunctions and nondisjunctions bring new relations in nuclear 

 organizations more varied or at least more intense than those of somatic 

 divisions. It seems clear that it is the interactions between chromosomes 

 or between parts of them that give rise to the degenerations in cell 

 processes and to the abortion and death of cells. The abnormalities 



