STOUT AND 8U8A, CHROMOSOME IRREGULARITIES 15 



When there are such high numbers, some of the units appear to be some- 

 what shorter than is usual (compare PI. I, Figs. 2 and 3 with 4 and 5), 

 but this is the only evidence which suggests that segmentation has oc- 

 curred in somatic divisions. There is definite evidence that the number of 

 entire chromosomes may increase to at least 19 by non-distribution. It 

 seems reasonable to consider that the numbers can increase further by 

 non-distribution to as many as 30, which is the highest number thus 

 far observed in a daughter nucleus in a somatic division (PI. I, Fig. 5). 



No evidence has been found of a completely arrested somatic division 

 in which two sets of daughter chromosomes combine in one nucleus to 

 give a double number, as described by Nemec (1903, 1904) for the 

 somatic cells of root tips treated with solutions of chloral hydrate. In 

 the root tips of the Europa clon the increases in the number of chromo- 

 somes arise primarily in non-distribution during the course of the forma- 

 tion of two nuclei. 



Thus far no direct and convincing evidence has been obtained which 

 shows that in plants of the Europa clon any of the pollen mother cells 

 receive more than 12 chromosomes. But following an apparently normal 

 synapsis, irregularities in the size, shape and number of chromatin 

 masses are very frequent. Part of this is due to non-pairing, in which 

 varying numbers of univalents are present with the limit of 12 when the 

 chromosomes are all intact and are in normal number. But the number 

 of chromatin units rises above 12 by (1) the failure of chromosomes to 

 develop fully during the early stages of diakenesis or preceding these 

 stages, (2) by fragmentation during diakenesis and in the prophases and 

 the metaphases of the first mitosis, (3) by the omission of pairing and 

 by the splitting of chromosomes in the first meiotic division until, in 

 the extreme, this division becomes the same as a somatic mitosis and 

 (4) by a second splitting in the second division. Thus the normal num- 

 ber of 24 chromosomes for the completed second meiotic division may be 

 increased to at least 60, although in the cases of the highest numbers 

 (PI. II, Figs. 24 and 30) some of the units may represent fragments of 

 chromosomes. 



The number of chromosomes or of chromatin units which nuclei re- 

 ceive in sporogenesis depends in part on variations in the division and 

 multiplication of chromosomes but also on irregularities in their dis- 

 tribution and in the number of nuclei that are formed. 



The distribution of the chromatin units is frequently unequal and 

 erratic in both the first and the second divisions of meiosis involving 

 non-disjunction and non-distribution, both when there is the normal 



