CYTOGENETIC ANALYSIS 499 



3. Each of the three types of large metacentric sex chromosomes easily identifiable 

 during meiosis and mitosis. 



4. New chromosomal forms arising spontaneously, or induced with agents, are 

 easily detected and their origin determined. 



5. Single-cell cloning facilitates the isolation of monosomic and mutant forms, as 

 well as assuring retention of the diploid or the desired parental type of lines. 



6. Quasi-diploid relationships can be visualized clearly whenever the diploid 

 number of chromosomes exists in particular types of cells. 



7. Heterochromatin of sex chromosomes and autosomes is clearly correlated to 

 variations in form and function (tritiated thymidine uptake). 



8. The diploid status can be retained for long periods of time (beyond one year) 

 in some sublines, having generation times of less than 14 hours. 



9. Monosomaty of the sex chromosomes {X x 0) is quite common. The X 2 and Y 

 chromosomes can be totally absent without imparting any deleterious effects on 

 viability of normal or malignant cells. 



Behavior of Chinese-hamster cells in tissue culture 



1 . Growth in various chemically defined media with whole serum (1-15 per cent) 

 is most favorable. 



2. Cellular generation times average 10-14 hours. 



3. Plating efficiency is quite high among a number of lines of cells derived from 

 normal and malignant tissue. 



4. Spinner culture adaptation has been successful with certain normal hypo- 

 tetraploid sublines. 



5. The great majority of cellular types are readily preserved in the frozen state. 



6. Viral susceptibility (polyoma, poliomyelitis, and measles) is moderate to high 

 for most lines tested to date. 



Current long-term derivatives in vitro 



1 . Adult fibroblasts from various organs. 



2. Fibroblasts derived from 18-day embryos. 



3. Transformed elements stemming from cells in the peritoneum following 

 irritation resulting from the injection of distilled water 24-48 hours earlier. (Aspira- 

 tion yields numerous cells without the necessity of sacrificing the animal.) 



4. Spontaneous and induced malignancies from representative normal and 

 diabetic strains. 



5. SE polyoma-induced sarcomas (produced in collaboration with Dr. Sarah E. 

 Stewart). 



6. Sarcomas and carcinomas arising from X irradiation and treatment with 

 carcinogenic agents. 



These derivatives have been cloned repeatedly in order to provide an extensive 

 array of karyotypes with which to conduct comparative interference microscopy, as 



