Lecture XXVIII. 255 



tain double the amount of germ-plasm contributed by each gamete 

 and as the number of chromosomes in each mitosis is equal to 

 that of the preceding one it follows that the amount of the germ- 

 plasm and the number of the chromosomes of the oosperm are 

 double those of each gamete. This consideration shows, if our 

 conclusions are correct, that there must be some point in the 

 line of cell divisions between the successive acts of fertilisation in 

 the sequent generations, where a reduction in the number of 

 chromosomes, and with it a reduction in the number of transmitted 

 characters, occurs. It is of peculiar interest that this reduction 

 was established only after its occurrence had been predicted on 

 theoretical grounds. 



In the higher plants the reduction-mitosis usually takes place 

 during the division of the sporogenous cells or the spore-mother- 

 cells. The large nucleus of these cells when preparing itself for 

 mitosis develops a particularly long and attenuated nuclear thread. 

 The length of this thread is very great, and it forms countless in- 

 tricate coils in the nuclear cavity. Gradually these coils resolve 

 themselves into a number of loops twisted more or less upon them- 

 selves. At this stage a curious and sudden change comes over the 

 nucleus. The thread, which has been up to this more or less 

 uniformly distributed through the nuclear cavity, is now swept 

 across it and forms a dense and tangled mass at one side, in close 

 contact with the large nucleolus. This stage, which is called 

 synapsis, lasts but a short time, and then the mass unravels gradu- 

 ally and the whole thread emerges in twisted loops. There is 

 some uncertainty whether the loops formed before synapsis are 

 identical with those which emerge after it. It is possible that the 

 thread forming the emerging loops is composed of two parts of the 

 original thread and that each emerging loop is thus fourfold of the 

 original thread. However this may be, the emerging thread breaks 

 into a number of twisted portions equal to half the number of the 

 chromosomes of the somatic cells of the plant ; e.g. if the nuclei 

 of the somatic cells contain twenty-four chromosomes, in this 

 mitosis twelve chromosomes only appear. Thus each of these is a 

 double chromosome, containing a double number of chromomeres. 

 After the separation of the chromosomes growth in thickness con- 

 tinues, apparently at the expense of the nucleoli which now 

 disappear. When the chromosomes have so far thickened that 

 their width is about half to a quarter of their length, the nuclear 

 membrane disappears and its cavity is invaded by the cytoplasm. 

 The achromatic spindle is formed and the short double chromo- 

 somes, having the form of pairs of twisted rods, loops or rings, 



