146 
Notes on Recent Literature. 
NOTES ON RECENT LITERATURE. 
CYTOLOGY. 
THE MODE OF PAIRING OF CHROMOSOMES 
IN MEIOSIS. 
T H E remarkably constant and characteristic phenomena associated 
with the reduction division in plants and animals, have long 
attracted attention, and the interpretations which have been suggested 
have been many and various. But whereas formerly these inter¬ 
pretations might be divided roughly into two groups, respectively 
affirming and denying the existence of a true reduction involving 
the qualitative sorting out of chromatin elements, the scene of 
the controversy has now shifted. It is now generally admitted that 
a qualitative separation does take place in the heterotypic division 
(or at any rate in one of the two divisions of the meiotic phase), but 
there is no such harmony as to the manner in which it is supposed 
that this end is attained. In the earlier form of the controversy 
attention was centred mainly on the observation of the changes 
involved in the actual heterotypic division, that is to say, on the 
late prophase and metaphase of this division ; it is now recognized 
that the true interpretation must rest on an accurate knowledge of 
the precise changes which take place at an earlier stage—the 
earliest stages of prophase. 
Broadly speaking, two'interpretations of the phenomena hold 
the field at the present day. The one, which can probably claim 
the greater number of supporters, involves the idea of a lateral 
pairing of the chromosomes, brought about by the approximation 
towards one another of two previously independent spireme threads, 
which are generally assumed to be derived respectively from the 
male and female germ-nuclei which united in fertilization. These 
two threads arrange themselves, during early prophase of the 
heterotypic division, or in some cases earlier, in parallel, becoming 
more and more closely approximated, until, at about the time of 
synapsis, a more or less complete, though temporary, fusion between 
the two takes place. From the contracted condition of the spireme 
characteristic of synapsis, there emerges a thread showing sooner 
or later distinct traces of its double origin. This double thread 
gives rise to the “bivalent ” chromosomes of the heterotypic division, 
that is to say, to pairs of somatic chromosomes which lie side by 
side, having been brought into this position by the approximation 
of the threads towards one another, which took place during 
prophase. The heterotype division consists in the separation from 
one another of the two members of each pair of chromosomes, one 
member passing to the one daughter nucleus, its fellow to the other. 
This process of separation takes place throughout the whole series 
of pairs of chromosomes, and thus a qualitative reduction is brought 
about, each of the daughter nuclei containing a single series of 
