42 NORMAL HISTOLOGY. 



which is characteristic of karyokinesis, and if the chromosomes are 

 really the carriers of hereditary peculiarities, this mode of division 

 can hardly favor their perfect transmission. 



The minute structures of the nucleus and cytoplasm described 

 have been studied in cells that were rapidly killed and then 

 stained. In the living cells these details cannot be detected, and it 

 is still uncertain whether they may not be in part due to the methods 

 employed in their study. If the superficial layer of a frog's cornea 

 be carefully stripped off and placed upon a slide in a drop of the 

 aqueous humor of the eye, covered with a cover-glass and then 

 quickly observed, the nuclei of the cells may be seen as round and 

 somewhat more highly refracting bodies than the surrounding cyto- 

 plasm. The nucleoli may also be made out, but the outlines of the 

 individual cells and the internal differentiation of the nucleus cannot 

 be seen. If the specimen be observed for a few minutes, there will 

 come a moment at which the whole picture changes. The nuclei 

 suddenly become distinct ; an intranuclear network appears and the 

 cell outlines are more sharply defined. These changes may be has- 

 tened by placing a drop of acetic acid at the edge of the cover-glass. 

 From this point the changes start and rapidly occur in the adjacent 

 cells until the whole specimen is affected. It is as though the definite 

 structures described in the foregoing parts of this chapter abruptly 

 crystallized out of a nearly homogeneous medium. These changes 

 probably mark the instant of death. The chromosomes present in 

 cells dividing by karyokinesis are visible in the living cells. The 

 intranuclear network of resting cells is not visible during the life of 

 the cell. It is not defmitelv known whether it and the minute 

 structures of the cytoplasm, as seen in dead cells, existed in the liv- 

 ing cells, or whether they w T ere formed by a species of coagulation at 

 the moment of death or were merely rendered more dense and there- 

 lore clearlv visible. 



