Page:Popular Science Monthly Volume 49.djvu/634

612 After the daughter nuclei are formed, the inclosing protoplasm divides between them, and thus there result two cells from the original one. In plants, their separation is commonly brought about by the formation of a distinct cellulose wall connecting with the original walls of the mother cell, and dividing the original compartment into two, and in most plants this wall begins to appear in the form of thickenings on the spindle fibers.

As we glance over this process of karyokinetic division just described, the phenomenon which must strike us as most significant is the longitudinal splitting of the chromosomes and the distribution of the resulting halves. Why should this exact halving of each chromosome and the invariable contribution of one half to each new nucleus be necessary? If it were merely important to divide the substance of a nucleus about equally between the nuclei derived from it, no such painfully exact method would be necessary; and, if unnecessary, we can not believe it would have been developed. Yet it is common to animals and plants of the most varying complexity of structure, and therefore doubtless of profound significance. Let us reflect that the cells of a given plant or animal possess and perpetuate the characters of that species. In case of many organisms, all or most of their cells are capable, under certain conditions, of reproducing the species to which they belong. And a given cell is always true to its kind. In other words, any cell possesses the hereditary characters of its species, which it has received from its mother cell and which it transmits to its descendants to the last generation. Two cells from two different organisms are, therefore, though indistinguishable in appearance, really as different as the organisms from which they were taken. The transmission of hereditary characters from cell to cell must, then, be definitely provided for. A little consideration will show that the evidence points at present distinctly to the nucleus as the probable seat of those characters in the cell. And, of the different constituents of the nucleus, no other has so distinct an individuality, or is so carefully divided between the daughter nuclei, as the substance of the chromosomes; while the evident need of a complete equipment of each nucleus with all the qualities of the species is quite met in its receiving an exact half of each chromosome. It is by no means proved, and it is perhaps not possible absolutely to prove, that the chromosomes are the material bearers of the hereditary characters of the species; but this view furnishes the best working hypothesis yet suggested as to the significance of the phenomena of karyokinosis. And it certainly correlates the concrete fact with the abstract problem in a most suggestive way.

In conclusion, it may be well to glance at some cell phenomena connected with reproduction. Allusion has already been made.