26 MARGARET C. FERGUSON 



longitudinal and transverse divisions of the skein are no longer 

 unintelligible. The sister-threads formed by the longitudinal 

 splitting not only unite again, but adjacent portions of the 

 double threads draw together and become more or less fused, 

 giving rise when transverse fission again becomes apparent 

 to the one half number of chromosomes. The forms of the 

 resultant chromosomes are exactly what would be expected 

 from such an origin. In fig. 18, b, for instance, adjacent 

 portions of double segments have fused at the ends, trans- 

 verse division has followed, and three chromosomes parallel 

 rods, a U, and a Y, are seen in the act of separation. When 

 the component chromosomes have fused at both ends only, the 

 ring, or, if a twist follows, the closed eight results ; if fusion 

 has occurred at but one extremity the V, U, or open eight is 

 formed ; if the segments remain attached at the middle point the 

 X occurs ; when the constituents of the double chromosomes 

 have united end to end and the bend has not taken place at the 

 point of their union the L results and so on. The structure or 

 composition of the X, Y and V forms of chromosomes as found in 

 plants have been explained in much the same way as the above 

 by Belajeff ('97 and '98), but he did not trace their development 

 from the closed spireme and considered these three forms as the 

 typical or characteristic ones whereas, in Pinus, the other forms 

 named have been quite as frequently observed. When the 

 chromosomes first become apparent, irregular fragments of the 

 chromatic substance are frequently left at various points (fig. 

 17), but these are ultimately absorbed, doubtless being appro- 

 priated by the growing chromosomes (fig. 20). l 



At the time when the chromosomes are being differentiated, 

 they often appear as if pulling away from the nucleoli, and may 

 be seen still connected with them by delicate threads (figs. 18, a 

 and c). The nucleoli now have a spongy or porous appearance 

 and fail almost absolutely to take either nucleolar or chromatic 

 stains. With the final separation of the chromosomes they dis- 

 appear altogether. The history of these nucleoli from the 

 primitive archesporium up to the time of their dissolution leads 

 irresistably to the conclusion that here, at least, there is a very 



1 See " Note " at close of Appendix. 



