The Gametogenesis of Nepa Cinerea ( Water Scorpion). 239 
and examination of the complete cycle of nuclear changes might 
explain the matter. The primary spermatocytes having increased 
in size undergo the characteristic nuclear changes in preparation 
for reduction division. There is a large darkly staining prominent 
body, the nucleolus, present, whilst the chromatin is distributed 
through the nuclear space in a tangled mass of lightly staining 
threads derived apparently from the irregular masses of chromatin, 
already mentioned, but as the number of these masses varies in 
different nuclei, and as it was impossible to follow the formation 
of the threads from them, it cannot be maintained definitely that 
the latter represent chromosomes in the resting stage as has been 
suggested (Wilson). The threads become finer and more distinct 
with still no definite arrangement until the loose ends of the 
threads appear concentrated on one pole. They are very numerous- 
and counting with any certainty is out of question. Thickening 
occurs, and the threads stain more deeply, whilst the number is 
reduced, due to pairing, although the actual process of fusion is 
difficult to follow. Meantime the tangled network straightens 
out, the threads shorten, and definite loops appear to form the 
“ bouquet ” stage. Contraction to the pole at which the threads 
first appeared follows, leaving the remainder of the nucleus clear. 
In the diplotene stage the thickened reduced filaments appear 
from the light mass of the contraction phase, displaying in some 
cases longitudinal cleavage with a tendency to form some sort of 
a ring, others only partially so, forming a ring at one end, and 
others just showing a slit. Here the nucleolus, which up to this 
stage had remained unchanged, appears to be attached to some of 
these threads. It is much reduced in size and irregular in outline. 
This stage of partially formed rings and attachments of the threads 
to the nucleolus has been noted in Notonecta. Eventually the 
haploid number of longitudinally split threads contract and become 
detached, whilst the nucleolus is unrecognizable. Transformation 
to tetrads follows, but I failed to find a satisfactory section of this 
stage ; in fact difficulty was experienced in clearing up these later 
stages. The chromosomes arrange themselves regularly along the 
spindle which has now been formed. Close examination of several 
of these spindles shewed the presence of an unpaired chromosome, 
and in the anaphase stage a chromosome is seen either in front of 
or behind the others, but usually the latter. This is evidently the 
hetero-chromosome, which it has been hitherto impossible to 
identify, for it is about the same size as the larger chromosomes 
of the diploid complex. After a careful examination of my pre- 
parations I have come to the conclusion that the hetero-chromo- 
some passes to the pole of the nucleus in the first spermatocyte 
division and divides equationally in the second, and further that 
it may precede or follow the autosomes when they converge to the 
poles after division ; and as regards its origin its behaviour has not 
R 2 
