580 VISIBLE CONSTRUCTIVE ACTIVITY IN PROTOPLASM. 



plant -with great constancy. In the same species the root-cap, stomata, and pollen- 

 grains are always found to be constructed on the same plan. In poppy ilowers 

 which hail developed two thousand years ago in Egyptian soil, and which were 

 then placed in tombs as ornaments of the dead, the cells of the anthers and pollen 

 are formed precisely as in poppy flowers which grow in our fields to-day. It is 

 important to hold firmly to the fact of this constancy. On it is founded not only 

 the possibility of distinguishing between species of plants, but, generally, the 

 conception of kind or species, to which we shall repeatedly return. 



The alteration of shape in the protoplasm and its walls, just described, refers in 

 each case really only to the external contour. Obviously, definite displacements 

 and arrangements in the interior of the living protoplasm lie at the foundation 

 of these alterations, and it is reserved for further investigation to establish these 

 latter as far as they are visible and recognizable. Hitherto the alterations 

 occurring in cell-division in the substance of the protoplasm, especially in the 

 so-called cell-nucleus, have alone been accurately observed, and what has been seen 

 there has already been briefly stated on a previous occasion. This is the place to 

 return to these remarkable phenomena, and to collect together the most important 

 results in a brief review. 



Let us look at a cell in which the protoplasm fills the whole interior. A large 

 cell-nucleus is visible in the centre of the cell-body. The protoplasm exhibits 

 when very highly magnified, granules, and fibrils, the latter long and short, curved 

 and straight, knotted and twisted or rolled into balls, and anastomosing into 

 a net-work. This structure appears most plainly, especially the filamentous 

 formation, in large nuclei. The twisted threads there visible have been termed 

 nuclear fibrils. In many instances there seems to be only a single much-twisted 

 thi-ead present in the nucleus. In other instances more are to be seen, and they 

 appear to be disti'ibuted with some uniformity in the nucleus, as shown in fig. 138 \ 

 The change begins first of all with the division of the nuclear threads; from them 

 are formed numerous short, twisted rod-like, or granular portions, which journey 

 towards the centre of the nucleus, take up a position there corresponding to the 

 equator of the cell-nucleus, which may be compared to a geographical globe (see fig. 

 138"), and arrange themselves into a plate which has been called the nuclear plate. 

 Soon, however, a detachment again occurs of the constituents of this plate. They 

 separate from each other, each fibril splitting into two, and seek the poles of the 

 spindle (fig. 138^). As they do so the fibrils turn and bend themselves, usually so 

 that those going one way have the form of a U, and in the other of n. Arrived at 

 the region of the pole, the filamentous portions fuse, contract on every side into a 

 dense skein (fig. 138*), and thus from one cell-nucleus two nuclei result. 



A system of very delicate filaments also plays a part in these movements of the 

 elements of the nuclear plate. These filaments, as may be seen in fig. 138 ^' ^- *■ form 

 a spindle. This spindle arises, not from the nucleus, but from the surrounding 

 protoplasm. The spindle appears to serve the nuclear fibrils for support and 

 guidance in their movements, leading the fibrils to the poles, where they join 



