﻿Hance : Pollen development in Zebrina pendula 65 



artefact, though it has been observed a number of times. In 

 Fig. 2 the smaller nucleolus is seen to have lost a great deal of its 

 staining capacity and is moving to the outer edge of the nucleus. 

 It undoubtedly degenerates as only the one large nucleolus is 

 found in succeeding stages. 



As the spireme begins to form, the chromatin appears in long 

 threads which completely fill the cavity shown in Fig. ?>. The 

 nucleolus stains less deeply but is still very distinct (Fig. 4) . 



Figs. 4-7 show various steps in synapsis. Fig. 5 shows the 

 cell developed to its maximum size. The chromatin threads of 

 Fig. 4 have grown to form thick chromatin bands. No foundation 

 has been found, after careful measurement of many different 

 stages of synapsis, for the theory that synapsis is due to a growth 

 of the cytoplasm away from the nucleus. The nucleolus is shown 

 in Figs. 6 and 7 (N). Fig. 7 shows the nucleolus beginning to 

 vacuolate, a process which always seems to precede its disappear- 

 As the chromatin bands come out of synapsis it is plainly 

 evident that they are paired — Fig. 8. In Fig. 9, in which the 

 paired bands are already splitting transversely to form the chromo- 

 somes, the paired nature is still more plainly visible. 



Metaphase. — In the polar views of the metaphase of the pollen 

 mother cells the chromosomes always appear in pairs. In counting 

 great care was taken to secure uncut cells. In a few cases certain 

 single chromosomes were counted as a pair when no other chromo- 

 some could be found near it which might be considered its mate. 

 It was believed that its mate was probably covered by itself. With 

 this method of counting Fig. 10 shows thirteen pairs of chromo- 

 somes, Fig. II, thirteen pairs, Figs. 12 and 13, twelve pairs each. 

 Fig. 18, which, if we judge from the size of the chromosomes and 

 their position, is the early anaphase of the heterotypic division 

 shows fourteen pairs of chromosomes and Fig. 14, which is a 

 similar stage, has fifteen pairs. 



In Fig. 15 the two chromosomes "X" have all the appearances 

 of heterochromosomes, but Figs. 16 and 17 show that any chromo- 

 some or chromosomes may behave in a peculiar way. In Fig. 16 

 we find an "odd " body (X and Y) going to each pole ahead of the 

 other chromosomes while Fig. 17 shows a pair trailing after the 



