112 



CELL-DIVISION 



2. Division of the Chromosomes 



In developing his theory of fibrillar contractility, Van Beneden 

 expressed the view — only, however, as a possibility- — that the 

 splitting of the chromosomes might be passively caused by the con- 

 tractions of the two sets of opposing spindle-fibres to which each is 

 attached.^ Later observations have demonstrated that this sugges- 

 tion cannot be sustained ; for in many cases the chromatin-thread 

 splits before division of the centrosome and the formation of the 

 achromatic figure — sometimes during the spireme-stage, or even in 

 the reticulum, while the nuclear membrane is still intact. Boveri 

 showed this to be the case in Ascaris, and a similar fact has been 

 observed by many observers since, both in plants and in animals. 



Fig. 54. — Nuclei in the spireme-stage. 



A. From the endosperm of the lily, showing true nucleoli. [Flemming.] 



B. Spermatocyte of salamander. Segmented double spireme-thread composed of chromo- 

 meres and completely split. Two centrosomes and central spindle at s. [Hermann.] 



C. Spireme-thread completely split, with six nucleoli. Endosperm of Fritillaria. [FLEM- 

 MING.] 



The splitting of the chromosomes is therefore, in Boveri's words, 

 ''an independent vital manifestation, ati act of rcprodnction'^on tJie part 

 of the chromosomes." ^ 



All of the recent researches in this field point to the conclusion 

 that this act of division must be referred to the fission of the 

 chromatin-granules or chromomeres of which the chromatin-thread 

 is built. These granules were first clearly described by Balbiani 

 ('76) in the chromatin-network of epithelial cells in the insect- 

 ovary, and he found that the spireme-thread arose by the linear 

 arrangement of these granules in a single row like a chain of bacte- 

 ria.^ Six years later Pfitzner ('82) added the interesting discovery 



1 ' 



87, p. 279. 



2 '88, p. 113. 



8 See '81, p. 63S. 



