2/6 THE PROTOZOA 



improbability of this theory, from the phylogenetic standpoint, was con- 

 vincingly shown by Boveri ('95); and Lauterborn ('96), admitting 

 this difficulty, proposed the theory in a slightly modified form. He 

 assumed that micronucleus and centrosome may have had a common 

 ancestor in some primitive bi-nucleated protozoon, such as Amoeba 

 binucleata (Schaudinn), and that intermediate stages may be seen in 

 certain existing Protozoa, such as Paramoeba eilhardi (Schaudinn) 

 and Noctiluca miliaris. He considered the micronucleus to have 

 arisen from one of these primitive nuclei, but by a differentiation 

 quite different from that which gave rise to a centrosome. This 

 theory, also, has been shown to be untenable by Boveri ('01). A 

 different theory elaborately worked out has appeared in several of 

 the recent publications of R. Hertwig ('95, '96, '98). In its latest 

 form, Hertwig's theory may be summarized as follows: (i) The 

 achromatic substance is at first uniformly distributed in the resting 

 nucleus, but appears during division as pole-plates, the analogue of 

 centrosomes ; (2) the achromatic substance becomes permanently an 

 intra-nuclear centrosome ; (3) it is extruded from the nucleus to form 

 an extra-nuclear centrosome. 



Still another point of view was suggested by Schaudinn ('96) after 

 his very remarkable observations and experiments on Acanthocystis 

 and Oxyrrhis manna. He suggested the following two possibilities: 

 Either the division-centre (his centrosome) is a structure formed within 

 the nucleus, becoming subsequently cytoplasmic in position, as it does 

 in the buds of Acanthocystis^ or as the nucleolus-centrosome does in 

 the nucleus of Oxyrrhis when immersed in dilute sea-water ; or else 

 it is normally cytoplasmic in position, and becomes secondarily intra- 

 nuclear. 



Not only in Protozoa, but also in Metazoa, the sphere and centro- 

 some have been described, in some cases, as coming from the nucleus. 

 The most noteworthy of these is Brauer's description of Ascaris mega- 

 locephala univalens^f)^] and Riickert's ('94) description of its nuclear 

 origin from the germinal vesicle of a copepod. 



In these two cases, however, it seems obvious that such an origin 

 among higher metazoan cells can have little value in problems relat- 

 ing to the historic origin of the centrosomes ; for here differentiation 

 is fully as complete as in any other cells, and the origin of such centro- 

 somes can give no light on the problem. To a certain extent, Hert- 

 wig's view, also, is open to the same criticism. 



Schaudinn's other alternative has opened the way for another view, 1 

 which, based upon the lower Flagellidia and Rhizopoda, explains 

 more primitive conditions in more primitive organisms. It is not 



iCf. Calkins ('98, '99). 



