MEIOSIS 209 



interkinesis, although a few anastomosing strands may have made 

 their appearance. An unexpected difference was found between 

 the megaspore and microspore formation, in that the microspore 

 mother-cell reached a volume more than twice that of the 

 megaspore mother-cell. 



The most important of the irregularities occurring at meiosis 

 were non-disjunction and double non-disjunction. Evidence for 

 both these phenomena was obtained at metaphase, and daughter 

 nuclei containing six and eight chromosomes were seen at inter- 

 kinesis and also during the homotypic division. It was found 

 impossible to estimate the frequency of such irregularities here. 

 This is more simple when pollen mother-cell development is being- 

 followed. 



The behaviour of the nucleus is thus almost identical in the 

 formation of the megaspore and microspore, the main difference 

 lying in size and shape. 



Sheffield has studied the chromosome behaviour in a number of 

 Fi hybrids between self-pollinated species of (Enothera, The 

 resting and earlier prophase stages of the pollen mother-cell nuclei 

 appeared to be the same in all species of (Enothera, as well as 

 mutants and hybrids, but as soon as the spireme emerged from 

 the second contraction knot, each was found to be characterised 

 by its own peculiar ring formation. 



In the case of the hybrid CE. eriensis X ammophila, when the 

 spireme emerged from the second contraction, the threads took 

 the form of a ring composed of twelve chromosomes with one free 

 pair. This arrangement persisted till anaphase, when the chromo- 

 somes which were adjacent in the ring drew apart and passed to 

 the opposite poles of the spindle. Although a ring of twelve 

 chromosomes occurred regularly in diakinesis, there was a distinct 

 tendency for it to break at one point and to give rise to a chain 

 before it reached metaphase. It was found difficult to determine 

 whether the break always took place at the same point in the ring. 

 The amount of non-disjunction was 9 per cent. 



CE. ammophila X eriensis. This reciprocal hybrid was quite 

 different, both morphologically and cytologically, from the above. 

 Here there is no pairing of the chromosomes during meiosis and 



