Notes on Recent Literature. 
147 
chromosomes, as compared with the paired or double series which 
is found in the somatic cells. 
The alternative interpretation provides the same final result, 
but involves a different method of pairing. It is a characteristic 
feature of the heterotype division, that the spireme, as it emerges 
from the contracted condition known as synapsis, is thrown into a 
series of loops. According to the interpretation we are now 
considering, the pairing of the chromosomes is brought about by 
the approximation towards one another of the limbs of these loops. 
Thus each pair of chromosomes is at first a U-shaped structure, 
the two chromosomes being joined end to end, the point of junction 
forming the bent portion of the U. The two parallel limbs of the 
U come closer and closer together, and sooner or later a transverse 
fission takes place at the angle, the paired chromosomes now lying 
side by side, ready for the separation which takes place in metaphase 
of the heterotype division. It must be noticed that this method of 
attaining the paired arrangement,—by the looping of the thread and 
the subsequent approximation of the limbs of the loops towards one 
another—involves an “ end-to-end pairing ” of the chromosomes. 
Since the loops are generally continuous with one another at their 
inner ends, it follows that the chromosomes are arranged at this 
stage in a single linear series. Although direct proof is necessarily 
lacking, the evidence is very strong in favour of the correctness of 
the hypothesis that the two chromosomes which are united in each 
pair are respectively of maternal and paternal origin ; it therefore 
follows that in the looped thread, as it emerges from its contracted 
state, chromosomes of maternal and paternal origin must be arranged 
alternately in a single linear series. 
This is the essential point of difference between the two views, 
for the one described first implies that the maternal and paternal 
chromosomes are arranged in two independent series, “ lateral 
pairing ” being brought about by the arrangement of the two threads 
parallel to one another, followed by an approximation which becomes 
so close (during or shortly after synapsis) that it amounts to a 
temporary fusion. 
At first sight it would appear to be a simple matter to decide 
between the claims of the two views ; for if the former is correct 
and the pairing is essentially lateral, the loops into which the 
spireme is thrown after synapsis should show signs of their origin 
from paired threads, in other words they should be double structures. 
In the majority of cases they can clearly be seen to be double; but 
this double appearance is explained, by the supporters of the end-to- 
end pairing, as being due to the precocious longitudinal fission of 
the thread in preparation for the second maturation division. 1 
1 It should be explained that this fission is described by all 
observers as taking place very early—at the latest it becomes 
apparent in the metaphase of the first division. Farmer and 
Moore look upon the heterotype division as “ the intercalation 
of a special form of chromosome-distribution during the 
course of what otherwise would not differ materially from an 
ordinary pre-meiotic mitosis.” (Quart. Journ. Microsc. Science 
48 , 1905, p. 548), and thus account for the fact that the 
longitudinal fission which prepares for the second mitosis 
has already appeared, before the intercalated mitosis is 
complete. 
