THE NUCLEUS 95 



plasm no less than in the nucleus. Biitschli (3) considers it possible 

 that the centrosome might have been originally a cytoplasmic 

 structure, which had nothing to do with the nucleus, but became 

 included in it when a nuclear membrane was formed. 



Attention must be drawn here to the remarkable genus Paramosba (Fig. 49) 

 founded by Schaudinn for the species P. eilhardi (see p. 228). In this form 

 there is present beside the nucleus a body which was termed the " Neben- 

 kern," consisting of a darkly-staining middle piece, at each end of which 

 is a cap of clear substance. The Nebenkern has generally been considered 

 to represent a centrosome, and Chatton (49) has put forward the suggestion 

 that it may correspond to a karyosome or a portion of a karyoaome that has 

 passed out of the nucleus with the centrosome. Recently, however, Janicki 

 (71'5) has described two new species of Parawce&a, and puts quite a different 

 interpretation upon the Nebenkern. He regards the middle piece as chro- 

 matin, the clear caps as archoplasmic masses, each of which contains a 

 centrosome ; and he considers the entire structure " as a second nucleus, 

 as it were, fixed in division, in which the state of division has become the 

 permanent form." He proposes to replace Schaudinn' s term " Nebenkern ' 

 by the term "nucleus secundus," and considers it especially comparable to 

 the " sphere " of Noctiluca (Fig. 65). Division of the nucleus and Nebenkern 

 takes place quite independently of one anothe'r. 



On the nucleolo-centrosomic theory, the whole karyosome with 

 the contained centriole, as found in many Protozoa, is compared 

 with the complex extranuclear centrosome of the higher organisms. 

 It is clear, however, that the karyosome consists chiefly of plastin 

 which is impregnated to a greater or less extent with chromatin, 

 and in which the centriole is imbedded. As Chatton (49) has 

 pointed out, the three elements which compose the karyosome are 

 independent of each other. When the centriole and chromatin 

 have left the karyosome, the plastin-mass remaining behind is 

 homologous in every way with the nucleolus of the metazoan cell, 

 and the only element common to both the karyosome of Protozoa 

 and the centrosome of Metazoa is the centriole. 



The nuclear theory of the centrosome is associated especially 

 with the names of Schaudinn and, in more recent times, of Hart- 

 mann and Prowazek (63). According to this view, the centrosome 

 represents a second cell-nucleus, and every cell is to be regarded 

 as primarily binucleate. The starting-point of the evolutionary 

 series would be such a form as Amceba binucleata, which possesses 

 two similar and equivalent nuclei. In the ne.x.t stage of evolution 

 one of the two nuclei became specialized more for kinetic, the other 

 for trophic, functions ; examples of this stage would be furnished 

 by Paramoeba (Fig. 49), with its nucleus and " Nebenkern," and by 

 a trypanosome, with its trophonucleus and kinetonucleus, the 

 Nebenkern of the first and the kinetonucleus of the second repre- 

 senting the kinetic nucleus. The central grain of the Heliozoa or 

 the extranuclear centrosome of the Metazoa would represent the 

 final stage of evolution, namely, a kinetic nucleus deprived of all 



