56 PROTOZOAN NUCLEUS 



Endomixis has been demonstrated in several races of Paramecium 

 aurelia by Woodrufi and Erdmann, as well as in another species of the 

 same genus. Fermor (1913) claimed to have seen the same process in 

 Stylonychia, while Calkins (1915 and 1916) described it in Didinium and 

 Uroleptus. The meaning of endomixis is not clear. That it takes place 

 quite apart from unfavourable conditions has been noted by Woodruff 

 (1925), who also proved that certain races of Paramecium in which it did 

 not occur died out. All that can be stated is that for the satisfactory con- 

 tinuation of the functions of the macronucleus, whatever these may be, it 

 seems necessary in many cases that this structure be renewed from time 

 to time. Though this usually takes place during conjugation, it may 

 occur at other times. An exception to this rule is afforded by the 

 behaviour of the ciliate Spafhidium spathula (p. 132). 



STRUCTURE OF THE NUCLEUS.— The nucleus of a Protozoon possesses 

 a nuclear membrane, which may be regarded as a special develop- 

 ment of the linin network of fibres or septa which traverse the enclosed 

 space (Fig. 1). The meshes of the network or spaces between the septa 

 are filled with a fluid substance known as nuclear sap. Distributed upon 

 the membrane or network as distinct granules or in one or more larger 

 masses is the chromatin material, while in most cases, somewhere on the 

 network, and most usually at or near the centre of the nucleus, is a body 

 known as the karyosome, which, on account of its affinity for certain stains, 

 has generally been regarded as consisting partly of chromatin and partly of 

 an achromatic substance {plastin). In some Protozoan nuclei the 

 karyosome does not seem to be present, but it appears in the nuclei of the 

 majority of forms. From what takes place in nuclear division, it appears that 

 the karyosome is composed mostly, if not entirely, of plastin material, and 

 that the chromatin of the nucleus is represented by the granules outside the 

 karyosome, for it is from them that the chromosomes are formed. Doflein 

 (1922), from a study of the nucleus of the flagellate Ochromonas granulans, 

 was led to believe that a true karyosome was devoid of chromatin, and 

 that during nuclear division it gave rise to the achromatic part of the 

 spindle, while the chromosomes were derived from the peripheral chromatin 

 which was situated outside the karyosome. On the other hand, Stern 

 (1924), from a study of the nuclear division of the Heliozoon Acanthocystis 

 aculeata, arrives at a conclusion which is the exact opposite of this. He 

 believes that the karyosome breaks up and gives rise to the chromosomes, 

 while the spindle is formed from the part of the nucleus between the 

 karyosome and the nuclear membrane. 



Sometimes several masses of plastin occur in a single nucleus, but it 

 seems doubtful if these should all be regarded as karyosomes. It is often 

 assumed that an intranuclear centrosome, the centriole, is present in the 



