6i2 POPULAR SCIENCE MONTHLY. 



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 origi- 

 nal 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 signifi- 

 cant is the longitudinal splitting of the chromosomes and the dis- 

 tribution 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 dif- 

 ferent organisms are, therefore, though indistinguishable in ap- 

 pearance, 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 considera- 

 tion 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 dis- 

 tinct 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. 



