410 University of California Publications in Agricultural Sciences [Vol. 2 



thin threads to short, thick rods. It also makes it more probable 

 that the chiasmas described by several authors and hitherto mainly 

 observed in animals may be the cytological evidence of crossing-over 

 phenomena met with in genetical investigations. The chiasmas are 

 ob.served in rather late prophases where the chromosomes are com- 

 paratively short and thick, and the fairly stable percentage of crossing 

 over between certain genes would hardly be explained through ex- 

 change between elements so thick as the chromosomes are in this stage. 

 But if the chiasmas are to be regarded only as evidence of an inter- 

 change or connection that has been accomplished before the chromo- 

 somes have been shortened by coiling up of the chromonemata, then it 

 is more intelligible. Actual crossing over will then take place at an 

 early stage but after the chromonema has been doubled to chromatids 

 within the chromosome. The chiasmas will remain as an evidence 

 of a crossing over that already has taken place, and the extraction of 

 the crossed-over chromatids, finally, will not take place before the 

 heterotypic anaphase. 



Chiasmas in plants similar to those just described in Crepis have 

 previously been shown to exist in Uvidaria (Belling, 1926), TuHpa 

 (Newton, 1927), and Hyacinthus (Belling, 1927). Newton dared 

 not insi.st upon any interchange of parts of the chromatids, as the 

 opening up of the four-strand group of chromatids in two different 

 planes might explain the peculiar appearance. But in Crepis it 

 hardly seems pos.sible to explain the appearance of some gemini, as 

 for instance the cross in plate 58, figure 8, in any other way than 

 by admitting a segmental interchange between chromatids of homo- 

 logous chromosomes. Here the outline of the chromosomes is seen 

 clearly and there is no question as to which chromosome the different 

 parts of the four chromatids belong. Thus far only one case of 

 genetical crossing over has been observed in Crepis (Collins, 1924, 

 p. 268). On the other hand Crepis is a favorable genus for a study 

 of the chromosomal mechanism, and the occurrence of a haploid 

 C. capillaris with only three chromosomes, and all three morpho- 

 logically different, opens up a new field for a study of the early 

 pha.ses of meio.sis, as Ilollingshead (1928) has pointed out in her 

 paper about the discovery of this haploid. 



Chodat (1925) described in Aliiiim a development of the hetero- 

 typic bivalent chromosomes from which he very ingeniously drew 

 the conclusion that interchange between homologous chromosomes 

 must take place, although in this case at the end of the chromosomes. 



