ZOOLOGY AND BOTANY, MIOBOSCOPY, ETC. 667 



of the chromatic granulations ; they are not really comprised in the 

 hyaloplasm, but probably contain a certain quantity of the chromatin 

 of the nuclear filament. On attaining a certain size they detach them- 

 selves from the folds of the filament ; their presence can only be 

 detected with certainty when their volume considerably exceeds that 

 of the largest of the chromatic granulations. Their function in the 

 life of the nucleus is at present unknown. 



The nuclear membrane is sometimes extremely delicate, as in the 

 endosperm, sometimes thicker, as in the large nucleus of the embryo- 

 sac. It is composed of a single layer of granulations, which may be 

 isolated from the cytoplasm. The currents between the nucleus and 

 the cytoplasm are the result of osmosis ; the cytoplasmic reticulum 

 and the framework of the nucleus are both in intimate connection 

 with this membrane. It is achromatic and derived from the 

 cytoplasm. 



As regards the chemical composition of the different parts of the 

 nucleus, they may be divided into two groups, although their exact 

 nature cannot at present be determined. The chromatic granulations 

 of the filament, which alone enter into the constitution of the nuclear 

 plate, contain the nuclein of Zaccharias, or the soluble nuclein (in 

 caustic soda) of Miescher. The hyaloplasm and nucleoli are com- 

 posed, according to Zaccharias, of insoluble nuclein or plastin. 



In the process of indirect division of the nucleus, Strasburger has 

 adopted a distinction into three phases : — prophasis, metaphasis, and 

 anaphasis ; Gui guard prefers to admit simply progressive and 

 regressive phases, with the separation of the elements of the nuclear 

 plate as the culminating point. 



The first change is usually to be observed in the nucleus itself ; 

 this is much less often preceded by a striation of the surrounding 

 cytoplasm. This change consists in the formation of the knot 

 (jpeloton), the folds of which gradually contract and thicken. The 

 nuclear membrane becomes more visible; and towards the end of this 

 stage two rows of chromatic granulations are sometimes seen in the 

 hyaloplasm of the filament. 



The segmentation of the filament takes place most often before, 

 less often after, the disappearance of the membrane. As soon as the 

 Bcgraents are formed, the doubling of the chromatic granulations 

 in the hyaloplasm makes itself manifest. At whatever time this 

 doubling takes place, the segments have always the form of a ribbon 

 at the moment when their longitudinal fission is about to take place in 

 the nuclear plate. In pollen-mother-cells and in the embryo-sac 

 tljcir halves remain recognizable after the formation of the two rows 

 of chromatic granulations. 



When the nuclear membrane is resorbed, the cytoplasm penetrates 

 into the nuclear fluid. The achromatic threads of the S2)indle are 

 tlien formed at the same time as the poles, whicli always uj)pear as if 

 Kituatcd, not in the interior of the nucleus, but in the cytoplasm itself. 

 The threads, which are continuous from (jne pole to the otlier, are 

 derived entirely from the cytoplasm in all eases in tlie vegetaljlo 

 kingdom which have yet been observed ; while in the salamander and 



