NUCLEAR DIVISION IN PROTOZOA. 219 



these nuclei, but along a different path from that taken by 

 the centrosome. A different theory has appeared elaborately 

 worked out in several of the recent publications of R. Hertwig. 

 In its latest form Hertwig's theory may be reduced to the 

 following three stages in evolution: (i) The achromatic sub- 

 stance is at first uniformly distributed in the resting nucleus, 

 but appears during division as pole plates, the equivalent of 

 centrosomes. 1 (2) The achromatic substance becomes perma- 

 nently an intranuclear centrosome. (3) It is protruded from the 

 nucleus to form an extranuclear centrosome. The centrosome 

 (i.e,, spheres) thus extruded becomes differentiated for other 

 functions in the micronucleus of Ciliata, while the centrosome 

 in Metazoa is defined as follows : " From a morphological stand- 

 point the centrosomes are to be considered as portions of 

 the achromatic substance of the nucleus which have become 

 extruded ; they are nuclear in origin. One can call them, in 

 a certain sense, nuclei without chromatin." Thus Hertwig has 

 introduced a new conception by regarding the centrosome as 

 coming originally from nuclear substance. Doflein substan- 

 tially follows Hertwig. 



Still another point of view was suggested by Schaudinn, 

 after his very remarkable observations and experiments upon 

 Acanthocystis and Oxyrrhis marina? 1 He postulated two possi- 

 bilities either the centrosome is a structure formed within the 

 nucleus, and subsequently becoming cytoplasmic, as it does in 

 the buds of Acanthocystis, and as the " nucleolus-centrosome " 

 does from the nucleus of OxyrrJiis marina when immersed in 

 dilute sea water, or else the centrosome is normally cytoplasmic 

 in position and becomes secondarily intranuclear. The latter 

 alternative is rejected by Hertwig as far less probable than 

 the former ; his criticism being based upon his own observa- 

 tions and experiments with sea-urchin eggs, and upon the 

 nuclear division of Actinosphcerium, in which he describes the 

 centrosome as formed from bits of chromatin. Brauer also 

 described centrosomes in Actinosphcerium, supposing them to 



1 Hertwig uses the term " centrosome " in the same sense that morphologists 

 in this country have used the term " sphere." 



2 Verh d. deutsch. zool. Ges., 1896. 



