PROTOPLASM AND LIFE. 735 



dependent individuality, and from this we should expect that, like all 

 living beings, it had the faculty of multiplying itself, and of becoming 

 the parent of other cells. This is truly the case, and the process of 

 cell-multiplication has of late years been studied, with the result of 

 adding largely to our knowledge of the phenomena of life. 



The labors of Strasburger, of Auerbach, of Oscar Hertwig, of 

 Eduard van Beneden, Btitschli, Fol, and others, here come prominently 

 before us, but neither the time at my disposal nor the purport of this 

 address will allow me to do more than call your attention to some of 

 the more striking results of their investigations. 



By far the most frequent mode of multiplication among cells 

 shows itself in a spontaneous division of the protoplasm into two sepa- 

 rate portions, which then become independent of one another, so that 

 instead of the single parent cell two new ones have made their appear- 

 ance. In this process the nucleus usually takes an important part. 

 Strasburger has studied it with great care in certain plant-cells, such 

 as the so-called " corpuscula " or " secondary embryo-sacs " of the 

 Conifera? and the cells of Spirogyra ; and has further shown a close 

 correspondence between cell division in animals and that in plants. 



It may be generally stated as the results of his observations on the 

 corpuscula of the Coniferce, that the nucleus of the cell about to divide 

 assumes a spindle-shape, and at the same time presents a peculiar 

 striated differentiation, as if it were composed of parallel filaments 

 reaching from end to end of the spindle. These filaments become 

 thickened in the middle, and there form by the approximation of the 

 thickened portions a transverse plate of protoplasm (the " nucleus- 

 plate "). This soon splits into two halves, which recede from one an- 

 other toward the poles of the spindle, traveling in this course along 

 the filaments, which remain continuous from end to end. When ar- 

 rived near the poles they form there two new nuclei, still connected 

 with one another by the intervening portion of the spindle. 



In the equator of this intervening portion there is now formed in a 

 similar way a second plate of protoplasm (the "cell-plate"), which, 

 extending to the walls of the dividing cell, cuts the whole protoplasm 

 into two halves, each half containing one of the newly-formed nuclei. 

 This partition plate is at first single, but it soon splits into two lamina?, 

 which become the apposed bounding surfaces of the two protoplasm 

 masses into which the mother cell has been divided. A wall of cellu- 

 lose is then all at once secerted between them, and the two daughter 

 cells are complete. 



It sometimes happens in the generation of cells that a young brood 

 of cells arises from the parent cell by what is called " free-cell forma- 

 tion." In this only a part of the protoplasm of the mother cell is used 

 up in the production .of the offspring. It is seen chiefly in the forma- 

 tion of the spores of the lower plants, in the first foundation of the 

 embryo in the higher, and in the formation of the endosperm a 



