418 DORIS L. MACKINNON. 
was generally much greater than at first appeared, for the 
cytoplasm was often reduced to a minimum by extreme 
vacuolisation and by the great thickness of the silica coat. 
The nuclear mass retained may be so much in excess of 
the available cytoplasm that development can proceed only 
up to a certain point. 
(a) Disintegration may occur in the primary cyst. Fig. 4 
shows one of a group of two primary cysts from the cold cul- 
ture of B8. The cyst pictured was of large size, 129 u X 79 n, 
but the silica was more than 10 thick, and the cytoplasm 
much vacuolated. So that the dimensions of the nuclei (the 
primary karyokinesis had already taken place), 19°54 X 19°8u 
in one case, and 18 X 22 1n the other, are excessive. The 
chromatin has been thrust out on all sides into the cytoplasm, 
and lies there in irregular strings, blotches and specks. ‘The 
largest chromidia do not radiate from the nuclei, but tend to 
lie closely apposed to their surfaces in a tangential direction. 
The nuclei are almost devoid of chromatin, and show signs 
of shrinking. Probably further development was impossible. 
Of the other cysts formed in this culture, one group made 
primary cysts with very large nuclei, and two more formed 
small vacuolated secondary cysts. The warm culture died off 
in the mother-cyst stage. 
(b) Fig. 5 represents a cyst from the cold side of culture 
Bll. Here are nine nuclei (only six appear in the section 
figured) of very uneven size,—one is 32m X 23p, others 
measure 26 X lop, 20m X 17 p, etc., and two are under the 
normal size for this stage, 16 4 and 15 respectively. They 
are all rich in chromatin, and show vacuolated nucleoli. In 
contrast to the available mass, the cytoplasm is so scanty as 
to be almost absent round some of the nuclei. Silica has 
been excessively developed, and in such a way as to indicate 
that an attempt has been made to map.out primary cysts, of 
which these nuclei are the centres. ‘The power to separate 
these definitely from one another has been lost, and further 
development is scarcely possible. Several of the nuclei are 
surrounded by chromatin haloes, in which chromidia radiate 
