ii2 PHYLUM PROTOZOA- -THE SIMPLEST ANIMALS. 



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. In the division of many Protozoa, as in 

 the cells of higher animals, it plays an important part. During division 

 it passes from the resting to the active condition. The nuclear threads, 

 or "chromatin filaments/' loosen themselves from their coiled state, 

 and arrange themselves in a star at the equator of the cell, whence they 

 divide into two groups, which retreat from one another, and become the 

 daughter nuclei of two daughter cells. In short, karyokinesis has been 

 observed here as elsewhere (see p. 48). 



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 Aiiiccba 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. There seems no reason why we may not combine the view of 

 Weismann, that the nucleus bears the essential hereditary substances, 

 with the view that it is a vital centre in the cell, and that the life of the 

 cell implies interactions between nucleoplasm and cytoplasm. 



In Gymnomyxa the outer part of the cell substance 

 ("ectoplasm") is often clearer and less granular than the 

 inner part ("endoplasrn"), but this difference is a physical 

 one of little importance. In corticate Protozoa there is a 

 more definite rind or thickened margin of cell substance. 

 Outside this there maybe 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. More- 

 over, the cuticle may become the basis of a shell formed 

 from foreign particles, or made by the animal itself of lime, 

 flint, or "horny" 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 already 

 mentioned. 



