494 GENETICS OF SOMATIC CELLS 



hydrolysate provides extra nutritional factors to help overcome toxicities and use of 

 unfavorable media for general subculturing purposes (Yerganian, unpublished data). 



(4) When pleuropneumonia-like organisms are suspected, substitution of 100 y of 

 Kanomycin sulfate (Bristol Laboratories) in place of penicillin and streptomycin for a 

 three-week period has been demonstrated by Pollock, Kenny, and Syverton 1009 to be 

 effective for well over six months. Retention of cultures at 41 ° C. for 4 hours has been 

 found by Hayflick 536 to destroy infectious agents selectively. 



5. Undertake trials of single-cell isolation for purposes of regaining the desired 

 types of cell only when adequate supplementary inspections can be made, such as 

 checking for chromosomal markers ; viral susceptibilities of the parental stock ; tumori- 

 genic potential in homologous or heterologous hosts; growth rate or cell-generation 

 time ; and colonial features, including distribution of heterochromatic masses and shape 

 of the nucleus. 



Plating efficiency may be improved by the addition of alpha-keto acids 941 and/or 

 the addition of 50 gamma of trypsin per ml. of medium. 322 



THE KARYOTYPE AND IDENTIFICATION OF INDIVIDUAL CHROMOSOMES 



The ever-increasing accuracy with which chromosomal complements of many- 

 rodents are being identified has been most encouraging (figures 54-64) . The human 

 karyotype is but one example of the manner in which heretofore numerically compli- 

 cated complements of chromosomes are being readily recognized today (figure 69). 

 Nevertheless, the greater the proportion of telocentric forms of chromosomes, the more 

 difficult it is to provide an accurate karyotypic analysis. Although the house mouse is 

 favorable for genetic studies, difficulties arise when attempts are made to identify the 

 individual chromosomes (including the sex elements) . Equally disturbing is the early 

 mitotic breakdown displayed by murine tissue cultures. 782 Renewed effort should be 

 made by tissue culturists to meet the challenge to control the karyotype of cultured cells 

 of the mouse and to reveal the true identity of the sex elements in mitosis. Currendy 

 only chromosomal behavior in the Chinese hamster is well understood, including its 

 complex triheterosomic somatic sex expression. However, this phase of cytology will 

 be in constant flux as reports on the karyotypic features of normally derived tissues 

 continue to appear. 



SEX CHROMOSOMES AND HETEROCHROMATIN 



The various terms that apply to the normal, yet exceptional, behavior of sex 

 chromosomes in the heterogametic sex (namely, the male) has recently regained 

 prominence (figure 59) . Further clarification of the phenomenon has become warranted 

 following the interesting revelations by Ohno and his collaborators. 963, 966 The 

 possible interrelationship between the sex chromatin of Barr and the actual sex chromo- 

 somes still is uncertain in man. Unfortunately, the presence of sex chromatin-like 



