2IO BIOLOGICAL LECTURES, 



some and astral rays but no distinct central spindle. (3) Forms 

 without centrosome and central spindle. 



In all of these cases the granular chromatin of the resting 

 nucleus is arranged in the form of a reticulum upon a linin 

 network, and the prophases of division are similar in that the 

 granules are brought together and concentrated in the form of a 

 much wound or coiled thread, — the spireme,' — while the stain- 

 ing reactions become much more intense. This stage is univer- 

 sally followed by segmentation of the spireme into chromosomes 

 of definite number, shape, and size for each species. Finally 

 the nuclear membrane disappears and the chromosomes are 

 left naked in the cytoplasm, but connected by spindle fibers 

 with the two poles of the mitotic figure. 



As the variations in the three types mentioned have mainly 

 to do with the achromatic structures, the chromatin changes 

 may be omitted. In the first type considered, the approach of 

 division is signalized by the division of the centrosome into 

 two daughter-centers connected by fibers whicH form a small 

 spindle, called the " central spindle " by Hermann ('89). 1 This 

 continually enlarges as the centrosomes diverge, other fibers 

 (mantle fibers) meanwhile growing from the centrosomes and 

 pushing in the nuclear membrane, which finally disappears, 

 leaving the chromosomes in the cytoplasm. The mantle fibers 

 then become attached to the chromosomes, and the latter 

 finally surround the central spindle like a ring. The origin of 

 these fibers is differently interpreted by different observers. 

 The mantle fibers, according to some, arise from cytoplasmic 

 material ; according to others, from linin substance in the 

 nucleus. The central spindle fibers, on the other hand, arise 

 from archoplasm (Boveri), or from the substance which sur- 

 rounds the centrosome (centrodesmus material of Heidenhain). 



Several transitional stages between the first and the second 

 types have been described by different observers. In these 

 the central spindle appears first as very delicate fibers between 

 the dividing centrosomes, but these break later and no connec- 

 tion remains. The complete spindle, in such a case, consists 

 of fibers which pass, apparently, from pole to pole, with chro- 



^ Arch.f. mikr. Anat., Bd. xxxiv. 



