A STUDY IN CHROMOSOME REDUCTION. 625 
Yamanoucai, S., 1906, ‘‘The Life History of Polysiphonia.” Bot. Gaz., xlii. p. 401, 1906. 
1908, ‘‘Sporogenesis in Nephrodium.” Bot. Gaz., xlv. p. 1, 1908. 
1910, “Chromosomes in Osmunda.” Bot. Gaz. xlix. p. 1, 1910. 
EXPLANATION OF FIGURES IN PLATES. 
All figures were drawn with the aid of the camera lucida and to the same scale x 1800, except figs. 
4, 5, 11, 12, 13, 26, and 27, which for clearness were drawn at a considerably higher magnification. 
Fig. 1. A section of a mature mother-cell with the nucleus in the so-called resting or reticulum stage. 
The chromatin is in the form of a network of interlacing delicate threads. 
Fig. 2. An older stage of the same. The chromatin threads are more sharply defined, and the distension 
of the nuclear cavity by the increased amount of karyolymph and the consequent withdrawal of the nuclear 
membrane from the chromatin mass are clearly indicated. 
Fig. 3. The same at a still older stage. The nuclear vacuole has greatly increased. The chromatin 
threads are shorter and thicker and show a distinct longitudinal fission. 
Fig. 4. A detail of the chromatin threads at a much higher magnification, showing the characteristic 
vacuolated nature of the longitudinal fission. 
Fig. 5. Another detail of the same at a slightly later period. 
Fig. 6. A tranverse section showing the growth period at about its maximum. The nuclear cavity has 
enlarged enormously and the chromatin threads have become very much shorter and thicker. The ends of 
the threads show very frequently and the longitudinal fission may be easily observed, 
_ Fig. 7. A somewhat older stage than that represented in fig. 6. The chromatin threads are not only 
shorter and thicker but show a distinct beaded appearance. The longitudinal fission of the chromosomes is 
very evident. 
Fig. 8. A slightly older stage of the same. Here it will be seen that the chromatin threads—still 
showing the longitudinal fission—have distributed themselves fairly uniformly throughout the nuclear cavity. 
After this stage the chromatin mass never has a lateral position. 
Fig. 9. This represents a section about the same stage as above, but not quite median. The longitudinal 
fission of the chromosomes is clearly shown, not only through the entire length in some cases, but the double 
nature of the ends is evident. : 
Fig. 10. Another case of the same. Here the chromosomes are so sharply defined and so uniformly 
distributed within the nuclear vacuole that an approximate estimation of their number may be obtained. 
Fig. 11. A more highly magnified detail of the chromosome, to show the vacuolated nature of the 
longitudinal fission at this time. 
Fig. 12. Another detail of the same. 
Fig. 13. Another detail of the same to show the incomplete separation of the two longitudinal halves of 
the chromosome ; also the double nature of the natural ends of the chromosomes. 
Fig. 14. A section that is not median. The longitudinal fission is very conspicuous. The beaded nature 
of the threads is not so evident. 
Fig. 15, A section that has been cut in a median plane. The large number of threads present makes it 
quite clear that the reduction in the number of chromosomes has not -yet taken place. The longitudinal 
fission is less evident. 
Fig. 16. A later stage. The spireme threads have become much shorter and thicker. They are 
Segregating in pairs preparatory to a lateral union. 
Fig. 17. The same a little later. The lateral pairing of the chromosomes is very evident. 
Fig. 18. The union of the pairing threads is more complete. 
Fig. 19. Another stage showing about the same condition of the pairing chromosomes. This and the 
preceding fig. show beyond any doubt that the union is a lateral one. 
Fig. 20. This fig. shows that the union of the univalent chromosomes is not only a longitudinal one, but 
that they coil closely about one another in a spiral fashion. 
TRANS. ROY. SOC. EDIN., VOL. XLVIII., PART III. (NO. 25). 91 
