392 The Unity of the Orgnnhtn 



Owing to the wide distribution, great abundance, and con- 

 spicuousness of NoctUuca, it has been a favorite animal for 

 study at the sea-side during many years, and its mode of 

 division has proved to be one of the most interesting fea- 

 tures about the creature. The most important observa- 

 tions on division in Noctiluca were made by Calkins (figures 

 35 a, b, c, d, e, taken from the original paper), and a few 

 quotations from Calkins' writings will set forth the cardinal 

 facts. "On the outside of the nucleus in Noctiluca, in the 

 cytoplasm and close against the nuclear membrane, is a 

 large, faintly staining spherical mass, which acts as a divi- 

 sion-center. During the early stages of nuclear activity, 

 the sphere divides into two similar halves, connected by a 

 strand composed of fibers w^hich are formed from the sub- 

 stance of the sphere. These fibers compose the central 

 spindle, and are homologous in every way with the central- 

 spindle fibers of the usual type of mitosis in Metazoa. The 

 nucleus then elongates in a direction at right angles to the 

 central spindle, and at the same time bends in the centre in 

 such a way that the central spindle sinks into a depression 

 in the nucleus, which surrounds it upon three sides. In 

 this w^ay the nuclear plate is finally wrapped about the 

 central spindle in the form of an incomplete ring. . . . 

 The nuclear membrane then disappears in that part of the 

 nucleus which is turned toward the central spindle, while 

 it is retained unbroken in all other parts of the nucleus. 

 Thus the chromosomes, as in the higher types, are brought 

 into contact with the central spindle fibres. They then split 

 longitudinally, and through the agency of the mantle-fibres 

 are separated into two equal groups, each group drawn 

 toward its own daughter-sphere. Within the sphere the 

 fibres are focussed in a centrosome which, at this period, 

 can be demonstrated with the greatest ease. The division 

 is finally completed by the separation of the remainder of 

 the nucleus and the re-formation of the daughter-nuclei. 



