2 INTRODUCTION. 



division of the nucleus. The first two of these processes will be dis- 

 cussed under nuclear division, while the third will be dealt with in 

 connection with cell-formation. 



NUCLEAR DIVISION. 



KARYOKINESIS IN CELLS OF THE LOWER PLANTS IN WHICH 

 CENTROSOMES AND CENTROSPHERES ARE DEVELOPED. 



At present there are recognized two types of development of the 

 karyokinetic spindle. In one the spindle arises through the instru- 

 mentality of individualized dynamic centers or centrospheres, as in 

 certain Thallophyta and Liverworts ; in the other, it is developed 

 wholly independently and in the absence of any such centers, as, for 

 example, in the higher plants. We speak of types of spindle develop- 

 ment in this connection also for the sake of convenience, since centro- 

 spheres have not been found in connection with the development of 

 the spindle in all Thallophytes ; but the author does maintain that 

 centrospheres have not been demonstrated to occur in any plant 

 above the Bryophytes, and that in the Angiosperms such structures 

 do not in all probability exist. 



As illustrating the development of the spindle in which centro- 

 spheres are present, the tetraspore mother-cell in Dictyota dichotoma 

 will be selected from the algae and the mother-cell of the ascus in 

 Erysiphe from the fungi. 



It is not considered necessary, nor conducive to any better under- 

 standing of the facts presented here, to enter into any lengthy dis- 

 cussion concerning the structure of the firmer framework of the 

 cytoplasm. The consensus of opinion now is that the firmer substance 

 of cytoplasm consists of either a reticulum of fibrillae or of an alveolar 

 or foam structure (Waben of German literature) and that, in many 

 cells, these two structures intergrade into one another. 



DICTYOTA. 



The cytoplasm of the tetraspore mother-cell of Dictyota dichotoma 

 during the preparation for nuclear division presents two well-defined 

 portions, the kinoplasm, which is always associated with the nucleus 

 and plays the most important r61e in the karyokinetic process, and the 

 remaining alveolar portion. Numerous chloroplasts are also present. 



The first indication of mitosis is the appearance, on opposite sides 

 of the nucleus, of two large sharply defined asters of kinoplasmic 

 fibers radiating from a rod-shaped body, which is often slightly bent, 

 lying either close to the nuclear membrane or at some little distance 

 from it (Fig. i, A). The rod-shaped body is the centrosome, which 



