8 BRITISH FRESHWATER RHIZOPODA. 
brane and a single compact mass of chromatin, which 
occupies the centre of the distinct nucleus and is separ- © 
ated from the membrane by hyaline matter. In others 
the chromatin reservoirs may be two or more, or there 
may bea great number of granules in the nucleus with- 
out the reservoirs (e.g. dincba proteus). ‘ In some of 
the Rhizopoda (Luglypha) and Heliozoa (dctinophrys 
and Actinospherium), the nucleus is strikingly similar 
to that of metazoan cells, consisting of chromatin in the 
form of a reticulum, and a network of linin.” * 
A true nucleus, comparable to the nuclear structure 
in Metazoa, according to Hertwig, apparently exists in 
no case, save possibly in Actinospheriwm, and even here 
it is limited to a passing phase during cell-division. 
Discussing this subject Calkins remarks on the prob- 
ability that the structures which have been almost 
universally, but erroneously, called nucleoh, do not 
belong at all to this category of nuclear elements, but 
represent either the functional chromatin which is 
aggregated into a central mass (karyosome) during the 
quiescent or vegetative period of cell-life, or the intra- 
nuclear division-centre. 
This author thus summarises the observations of 
Gruber, Hertwig, Brauer, and others upon the pheno- 
mena presented by the nucleus, during mitosis (cell- 
division), and concludes: “The facts point toward the 
conclusion that the centre of activity in the division of 
the protozoan cell, as in Metazoa, resides in a special 
structure, which, to avoid confusion in terminology, 
has been called the division-centre. In some cases this 
structure resembles the astral system of Metozoa, in’ 
consisting of an outer spherical mass with radiating 
processes (astrosphere), and an inner focal granule or 
granules (centrosome). The evidence further tends to 
show that the division-centre in Protozoa consists of 
a specific substance different from the chromatin and 
from the cytoplasm, and possessing above all other 
portions of the cell an active ;d/+ in division. No con- 
* Calkins, op. cit., p. 87. 
