754 CALKINS. [Vol. XV. 



In SpirocJiona the so-called " centrosome " passes from the 

 nuclear chromatin into the achromatic body, which, however, 

 still lies within the nuclear membrane. It is possibly nuclear 

 in origin in Actinosp/iaerium, becoming extra-nuclear during the 

 metaphase of division. It appears to be nuclear in origin in 

 Noctiluca, but is found in the cytoplasm in the achromatic 

 body in the early metaphase and remains permanently in the 

 cytoplasm in spore-forming individuals. Finally it is perma- 

 nently cytoplasmic in the Metazoa, although occasionally nuclear 

 in origin even here {Ascaris megalocephala univalens). 



The important position that Noctiluca must hold in all theo- 

 ries of mitotic development has not hitherto been sufficiently 

 emphasized. Much stress, indeed, has been laid on its simi- 

 larity in mitosis to metazoan cells, but little has been done to 

 show its relations to other Protozoa and the origin of its mitotic 

 structures. When considered alone, Noctiluca can throw but 

 little light on the origin of the complex elements of the meta- 

 zoan mitotic-figure, but when considered in connection with 

 other Protozoa the origin of its own elements is seen, and so, 

 indirectly, the probable origin of the elements in Metazoa. 

 Noctiluca thus holds an intermediate position in the probable 

 development of mitosis, and the inference may be drawn that 

 the origin of the mitotic elements in Metazoa is the same as in 

 Noctiluca . 



The position which Noctiluca holds in the development of 

 chromatic structures has been sufficiently pointed out. It has 

 been shown, first, that Noctiluca has probably the most primi- 

 tive form of true chromosomes ; and, second, that Noctiluca 

 presents probably the most primitive ring-like arrangement of 

 the chromosomes and nuclear plate around the central-spindle. 

 In regard to the "achromatic" structures the position of 

 Noctiluca is not quite so definite, and to be understood must be 

 considered in connection with lower forms. It has been main- 

 it originates from chromatin in the region of the " Zwischenkorper." Further 

 work on this questionable structure must be done before its proper position can be 

 determined ; that it is an important element in the cell and has a role to play in 

 nuclear division is established beyond question by the independent observations 

 of Hertwig, Plate, Biitschli, and Balbiani. 



