An Electron Microscope Study of Isolated Chromosomes.

HE limitation in the penetrating power of electrons is one of the most T serious obstacles in the study of chromosomes by electron microscopy. Chromosomes are generally larger than 0.2p in diameter and are nearly opaque to electron penetration. At various stages of the nuclear cycle, however, chromosomes change their size and shape. Dividing chromosomes are generally largest and “resting” chromosomes smallest. In order to take advantage of the small size of resting chromosomes, this study has utilized chromosomes from resting cells. A second serious obstacle to the use of electron microscopy in the cytology of chromosomes is the need for drying the material. In the process of drying, changes take place in the chemical and physical nature of the medium which surrounds the chromosomes and this seems inevitably to alter their structure. Air drying has not been found practical without previous cytological “fixation.” The method of freeze-drying has been tried but has not been found practical. In the air-drying of fixed material, some slight change of surface form seems inevitable, such as ripple-marks made by the decreasing size of the drop of water upon the soft surface of the chromosome. The internal structure of chromosomes as indicated by genetic deduction and cytological observation suggests a finer structure which might be discernible by electron microscopy. The opaque nature of the chromosome, due to its large size and impregnation with fixatives, compels one to study the finer structure by means of controlled decomposition of the body.