NOTES ON THE STRUCTURE OF PROTOZOA. 115 
(2) In the majority of cases, notably in the Sporozoa, the nucleus 
is single, often large, and placed centrally. From a consideration of the 
cells of Metazoa we may call this the typical case. 
_(3) In many of the Ciliata, e.g. Paramecium, there are two dimorphic 
nuclei. There is a large oblong nucleus, and beside it a smaller 
spherical one. he 
(4) In some Ciliata the macronucleus exists in the form of powder 
scattered through the protoplasm, ¢.g. in Ofalinopsds. The granules 
may collect to form a compact nucleus when fission is about to take 
place. 
(5) In Ofalina, from the intestine of the frog, and a few other forms, 
there are very numerous nuclei, arranged in a symmetrical manner in 
the cell substance. In some cases these isolated nuclei have been 
observed to unite to form one large nucleus just before binary fission 
takes. place. Of these various cases the diffuse condition is apparently 
very primitive. , 
The nucleus, when stained and examined under high powers, is 
observed to be complex in structure. It consists of a nuclear network, 
or a coil of chromatin threads. Karyokinesis has been observed in 
some cases. 
While we cannot at present define the physiological import of the 
nucleus, we must recognise its importance. Thus Bruno Hofer has 
shown that when an Amada is cut in two, the part with the nucleus 
lives and grows normally, while the part without any nucleus sooner or 
later dies ; and Balbiani has observed that in the case of Infusorians cut 
into pieces, those parts which have nuclei survive, while if no nucleus is 
present in the fragment, the wound may remain unhealed, and death 
ensues. 
The outer part of the cell substance (‘ectoplasm”) is 
often clearer and less granular than the inner part (‘‘ endo- 
plasm”). In corticate Protozoa there is a more definite 
rind or thickened margin of cell substance. Outside this 
there may be a “cuticle” distinct from the living matter, 
sometimes consisting of chitin, or gelatin, or rarely of 
cellulose. The cuticle may form a cyst, which is either a 
protection during drought, or a sheath within which the 
unit proceeds to divide into numerous spores. Moreover, 
the cuticle may become the basis of a shell formed from 
foreign particles, or made by the animal itself of lime, flint, 
or organic material. 
In the cell substance there may be bubbles of water taken 
in with food particles (food vacuoles), contractile vacuoles, 
fibres which seem to be specially contractile (in Gregarines), 
spicules of flint or threads of horn-like material, which may 
build up a connected framework, and the pigments raealdy 
mentioned. 
