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



16-18). A corresponding difference between chromosomes cannot be 

 seen during meiosis of C. aspera (pi. ^^S, figs. .'5-11). The pairing 

 between the homologous chromosomes seems to be perfect in these two 

 pure species, no irregularities being observed. As for the study of 

 development of the gemini and structure of the chromosomes, certain 

 species of Crepis are very favorable, the chromosomes being faii'ly 

 large, of characteristic shape, and their numbers few. By using the 

 described combination of Carnoy's and Navashin's fixatives and stain- 

 ing with iodine-gentian violet-orange, a very good and clear picture 

 was obtained. 



In early prophase single and rather thin threads of chromatin are 

 seen in the nucleus (pi. 58, fig. 1). It is not a continuous spireme, as 

 free ends are found, but even Avitli the small number of 8 chromo- 

 somes in the cell it is difficult to determine the number of free ends, 

 especially since the sections were differentiated considerably in order 

 to show the structure of the chromosomes. The thread appears 

 thickened at small intervals thus giving the appearance of a string 

 of beads. Apparently these thickenings are real morphological unitfi 

 of the chromosome, namely, the chromomeres. In the stage shown in 

 figure 1 the thread and the chromomeres probably are double, united 

 in parallel pairs. This seems evident partly from the fact that the 

 thread and the chromomeres are thicker here than in the early diplo- 

 phase shown in plate 58, figure 2h, and partly because the thread in 

 a stage a little later than figure 1 opens up and shows double in a 

 few places (pi. 58, fig. 2a). The stage depicted in figure 1, therefore, 

 is presumably to be regarded as a zygopkase with the chromomeres 

 from the parallel homologous chromosomes united tAvo and two. 

 Some places give the impression that the chromomeres are joined end 

 to end b,y double threads. 



In the diplophase four double units are seen in each of the two 

 species (pi. 58, figs. 2-9, 12-13). To begin with, the partners of the 

 gemini are somewhat irregularly twisted around each other .several, 

 up to 5-6, times (pi. 58, figs. 2-3). That each of these units rep- 

 resents a homologous pair of chromosomes is evident from the number, 

 which is four. Thus the two constituent members of a given unit 

 cannot be explained as the split halves of one chromosome but must 

 necessarily be one of the partners of a conjugated pair of chromo- 

 .somes. Parasyndesis is unquestionable in this case. 



A little later the chromatin thread of the chromosome coils up 

 into a more or less regular spiral, as one of the chromosome pairs in 



