Notes on Recent Literature. 
150 
resting nucleus of the somatic cells, two parallel spireme threads 
can be recognized. These two threads run through a reticulate 
framework consisting of fine threads, probably of linin, which 
contain no chromatin and have not been observed to connect with 
the spireme threads (Op. cit.,p. 22). Similar structures with a similar 
arrangement are found in the nuclei of the germ-cells of Ricliardia, 
though in this plant the parallel arrangement of the connecting 
filaments of linin is not so easily made out. During the prophases 
of the heterotype division these parallel spireme threads become 
more distinct, the synaptic contraction occurs, and shortly afterwads 
the closest approximation of the parallel bodies to one another 
takes place. That is to say, the pairing of chromosomes, which is 
characteristic of the heterotype division, is already foreshadowed in 
the paired arrangement of the chromosomes in the somatic cells of 
these plants. 
As Overton says, it is not easy to see how such observations 
can be related to a method of pairing, brought about by the folding 
of a thread into a series of loops. The later stages of the preparation 
for the heterotype division might be susceptible of interpretation on 
this view, but the earlier stages and the somatic nuclei, as described 
by Overton, are something of a stumbling block. In the search for 
a correct interpretation of the phenomena involved in the heterotype 
division, the point upon which attention has been especially 
concentrated has been shifted from the late prophase and metaphase 
to the earliest prophase ; are we to find the answer in a stage 
earlier still—in the structure of the somatic nuclei ? 
We have still to reckon, however, with Farmer and Moore’s 
observations upon Periplaneta. 1 As has been mentioned, in the 
pre-meiotic nuclei of this genus, double structures appear, which, as 
a comparison of the figures will show, are very like the paired 
prochromosomes described by Overton. Farmer and Moore look 
upon these as the “ Anlagen ” of single chromosomes, in which the 
longitudinal fission appears thus early in the prophase of the division, 
and they give a description of the prophase of the heterotype 
division which differs fundamentally from that of Overton. They 
find that in this division, instead of the chromosomes making their 
appearance in the form of thirty-two split rods, as they do in the 
pre-meiotic mitoses, the spireme thread segments into about half 
as many lengths (which of course represent pairs of somatic 
chromosomes joined end to end). These lengths are bent round 
upon themselves in the form of loops or rings ; the loops break 
across the curved portion and give rise to the pairs of chromosomes 
which now lie side by side. 
Although some of the stages figured by Farmer and Moore, in 
the prophase of the heterotype division, do not find any quite exact 
parallel in Overton’s figures of Thalictrum and Calycanthus, the 
divergence of opinion is no doubt primarily connected with the 
different interpretations put upon the double structures found in 
the pre-meiotic divisions. According to Farmer and Moore the 
number of these double structures would be the same as that of the 
somatic chromosomes ; according to Overton each double body 
consists of the primordia of a pair of somatic chromosomes. The 
Q. J. M. S. 48, 1905, pp, 528-537. 
