458 Sargant . — The Formation of the 
as the nucleolus is often displaced and broken by the knife. 
Tangential microtome sections however give details of the 
structure of the chromatic thread very well in places. 
Two periods of synapsis may be distinguished. During the 
first the chromatic ribbon grows greatly in width but the 
dots continue small. (Cf. Fig. 13 a with Fig. 13#: 13^ 
is drawn from a nucleus which has just entered the first 
period of synapsis, and 13 b from the nucleus figured at 13 
which is near the end of the same period.) This early period 
is marked by a large and washy nucleolus of irregular outline 
and by the ill-defined nuclear membrane. In the second 
period the nucleolus is well defined and spherical, and the 
nuclear membrane reappears. The ribbon does not become 
wider, but the dots which border it increase in size. The 
coils of ribbon also become looser and occupy a larger part 
of the nuclear cavity. Insensibly the structure of the nucleus 
becomes that of the well-known spirem stage (Fig. 14. Cf. 
also Guignard’s Figures 44-47, l.c.). 
We have now traced the formation of the spirem thread in 
great detail. The linin ribbon bordered on either margin by 
a row of chromatin granules can be followed back to the 
beginning of synapsis. There can be little doubt that this 
double row is formed just before contraction of the chromatic 
thread by the fission of the pre-existing single row (cf. Figs. 
it and 12). The linin ribbon at this period and during early 
synapsis seems to be fed from the partially dissolved nucle- 
olus. I am inclined to think that the solution of the nucleolus 
is brought about by the entrance into the nuclear cavity of 
some liquid from the cytoplasm, otherwise it is difficult to 
explain the temporary disappearance of the nuclear mem- 
brane. The contraction of the chromatic coils round the 
nucleolus is possibly in order that the interstices may be too 
small to admit of the escape of semi-fluid nucleolar matter. 
We shall meet with some of these phenomena again at a 
later stage. 
The structure of the nucleus in the spirem stage is too 
well known to need a long description. In hand sections 
