148 BULLETIN OF THE 



by a simple constriction, as in direct nuclear division. This fact has led 

 Waldeyer ('88) to the conclusion that kai'yokinesis in based upon the 

 simple scheme of division conceived by Remak. He says : " I would 

 interpret the facts in such a way that we have to regard as the funda- 

 mental form the simple amitotic division, which is now proved for many 

 cases ; it always takes place where the nucleus either is poor in chroma- 

 tin, or when it does not matter about strict bipartition of the chromatic 

 material. Should the latter be required, then we shall find mitosis, 

 since it is the most direct, most certain, and most simple manner in 

 which an exact bipartition of chromatic substance is brought about." 



It seems to me, however, that there are differences of so fundamental 

 a character between mitosis and amitosis, as at present understood, 

 that it is impossible to refer them to a single plan of division. Both, 

 indeed, achieve the same result, — division of the nucleus, including its 

 two constituents, chromatin and karyoplasm. In both cases, the karyo- 

 plasm divides by constriction. In amitosis, the chromatin undergoes 

 little if any change in preparation for division ; in mitosis it becomes con- 

 solidated into a limited number of thickened rods or loops (chromosomes), 

 which arrange themselves in the plane of division (" mother star," 

 " couronne ^quatoriale ") and segment either longitudinally or trans- 

 versely, the halves moving to opposite poles (" diaster "), and undergo- 

 ing a reversed metamorphosis to form two daughter nuclei. If this 

 were all there is to karyokinesis, — and in some cases the process is 

 much simpler, — we might hope to find transitions between it and ami- 

 tosis ; for there are examples of amitosis in which the chromatic net- 

 work undergoes changes during division, and it would be conceivable 

 that the highly organized changes of the chromatic substance during 

 mitosis were either evolved from them, or that they were a simplifica- 

 tion of the more detailed changes. In mitosis, however, other struc- 

 tures besides chromosomes make their appearance, — the centrosomes, 

 attraction spheres, and spindle. These structures are not known to take 

 any part whatever in amitosis, and in this respect at least the two kinds 

 of division are fundamentally different. The most recent workers upon 

 karyokinesis agree in assigning to the spindle rays the function of 

 separating or dividing the chromosomes, and drawing {or pushing) the seg- 

 ments towards the poles. The centrosomes are focal points towards which 

 the spindle rays converge, and lie entirely outside the nucleus. The for- 

 mation of the spindle has been carefully studied by many investigators 

 of karyokinesis, and, while there are very divergent views as to its ori- 

 gin and mode of action, the most recent workers in this field (of whom 



