STOUT AND SUSA, CHROMOSOME IRREGULARITIES 19 



Two splittings of single chromosomes during meiosis, such as are seen 

 in Europa are apparently not frequent among plants. Such behavior 

 has been reported especially for certain Hieracium hybrids by Rosenberg 

 (1917) and for certain canina roses by Tackholm (1922). Karpechenko 

 (1927 a and b) also reports a splitting of some of the univalents in the 

 first division as well as in the second division, but he states that this 

 very seldom occurs in his material. Karpechenko's studies are of special 

 interest as reporting definitely the development of functional gametes 

 with the somatic number of chromosomes or with higher numbers. In 

 his 1\ hybrids between Raphanus sativus x Brassica oleracca the somatic 

 number is 18 with 9 derived from each parent. Most frequently the 

 number of chromosomes in gametes is 9 but these abort or are non- 

 functional. Only a few spores are functional and these have 18 or more 

 chromosomes — the numbers 18 and 36 being most frequent. The number 

 18 arises by the complete omission of a reduction division, followed by 

 the organization of all the chromosomes into one nucleus, which divides 

 in the second division of sporogenesis simply as a somatic division, giv- 

 ing two spores with 18 chromosomes each or in case of non-distribution 

 with such numbers as 16 and 20. 



Karpechenko also reports that pollen mother cells may contain two 

 nuclei, whose chromosomes collect in one spindle, divide as in somatic 

 divisions and give rise to two pollen grains with 36 chromosomes in 

 each. This seems to suggest that the chromosome groups from the male 

 and female parents of these hybrids have maintained group identity and 

 not mingled as they usually do. One of Karpechenko's figures (his Fig. 

 17, 1927a) shows more than 70 chromosomes, a condition which he con- 

 sidered to be due to a "two-fold splitting of some univalents" or a split- 

 ting in the first as well as in the second division of meiosis. By such 

 means the number of chromosomes is multiplied in certain cases to at 

 least twice the somatic number. These extra splittings parallel those we 

 have observed but we have seen no indication that two nuclei are present 

 in the pollen mother cells of plants of the Europa clon. 



Piech and Moldenhawer (1927), also working with hybrids between 

 Raphanus sativus and Brassica oleracea but evidently with different 

 strains, find an increase in the number of chromosomes during the first 

 meiosis from 18 to as many as 36 by the splitting of univalents. They 

 report no further splitting and they state that in the second division four 

 spores are formed and that the number of chromosomes is thus reduced to 

 9. The second division is hence a reduction division. But irregular dis- 

 tribution or the formation of fewer than four spores may give more than 



