268 THE PROTOZOA 



the origin of the central spindle from the substance of the sphere, 

 are all features obviously common to the analogous structures in 

 Metazoa. 



An extra-nuclear body, somewhat similar to the sphere of Noctiluca> 

 has been described by Schaudinn ('96) in the rhizopod Paramceba. 

 Paramoeba reproduces by swarm-spores (cf. p. 93), which are formed 

 by the spontaneous division of the parent organism into a large num- 

 ber of small parts. Before this multiple division, the extra-nuclear 

 achromatic mass (division-centre) divides into a number of parts equal 

 to the number of spores to be formed, and after the several portions 

 are distributed about the cell, the nucleus divides into as many parts 

 as there are portions of the achromatic mass. It is unfortunate that 

 the minutiae of division are not given, and until future observation 

 confirms Schaudinn's interpretation, his results must be received with 

 some scepticism. The swarm-spores themselves reproduce by longi- 

 tudinal division, and the nuclear processes involved are extremely 

 suggestive of the relations of the extra-nuclear to the intra-nuclear 

 division-centres. 



3. The Relation of Extra-nuclear to Intra-nuclear Division-centres. 



In view of the fact that the division-centres are in some cases 

 extra-nuclear and in others intra-nuclear, the question naturally sug- 

 gests itself as to the connection, if any, between them. No one 

 conversant with the facts will doubt that they are analogous struc- 

 tures, but that one has been derived from the other is not so obvious. 

 Hertwig ('95) held that "these centrosomes of the egg-cell (Echi- 

 noderms) are not specific cell-organs, but portions of the nucleus 

 which have become freed from the chromatic nuclear substance." 1 

 This view of the origin of extra-nuclear kinetic substance in Metazoa 

 is difficult to accept, and in forming it, Hertwig passed over too many 

 questionable intermediate stages. There is considerable evidence, 

 however, among the Protozoa, to indicate that Hertwig's conception 

 has a basis of fact, and that the extra-nuclear division-centres arose 

 from the intra-nuclear forms. 



As indirect evidence of such an origin of the extra-nuclear division- 

 centres, it might be pointed out that in all mitotic figures in which 

 there is a central spindle, a portion, at least, of the spindle substance 

 is surrounded by chromatin, and may be said to be intra-nuclear in 

 position. This is certainly the case in all Protozoa, and the relations 

 of the extra-nuclear centres to the chromatin is particularly sug- 

 gestive in the dividing swarm-spores of Param&ba, in Noctilnca, in 

 Tetramitus, and in Actinosphcerium, while, according to Schaudinn's 



1 Loc. ciL y p. 53. 



