42 



AN INTRODUCTION TO MODERN GENETICS 



at the end of a division they are orientated with their attachment con- 

 strictions towards one side of the nucleus (the so-called pole-field) 

 which may often be recognized either by its position within an asym- 

 metrical nucleus or by the position of the nucleus in the cell. When the 

 chromosomes appear again in prophase they are still polarized, with 

 their centromeres concentrated in this region. A similar polarization is 

 often found in the prophase of meiosis, when the zygotene pairing 

 starts in the pole-field and progresses over the chromosomes from there 

 (this is the so-called bouquet stage, an exaggeration of which, particu- 



Fig. 8. Persistence of Chromosome 

 Arrangement through Interphase. — A 



Anaphase in the Coccidian Aggregata; only 

 some of the chromosomes are indicated. 

 6 interphase supervening shortly after the 

 stage drawn in A. C the anaphase of the next 

 division, showing the long chromosomes, 

 which are now separating towards the top 

 and bottom of the page, still not entirely free 

 from their partners in the division before. 

 (After Belar.) 



larly under conditions of bad fixation, leads to a collapse of the chro- 

 mosomes to one side of the nucleus, a phenomenon which is called 

 synizesis). 



A particularly clear example of the continuity of the chromosomes 

 was given by Boveri (1909), who investigated the nuclei of the first 

 blastomeres of the eggs of Ascaris megalocephala var. ufiivalens, which 

 contains only one pair of chromosomes. At a telophase the four ends of 

 the chromosomes project from the surface of the two daughter nuclei, 

 forming cylindrical protruberances; the two nuclei are mirror images 

 of one another. The characteristic patterns of protuberances and 

 associated chromosomes which are found in telophases are also found 

 in prophase, suggesting that the chromosomes have persisted in some 

 way within the protruberances and have appeared again in the same 

 positions as they occupied when they disappeared from view. In 

 Aggregata eberthi (Protozoa) Belar^ found that the interphases in the 



^ Belar 1926. 



