184 CELLS, TISSUES, AND ORGANISMS 



the analysis of their biochemical activities (Puck and Fisher, 1956; 

 Puck, 1958a). The actions of a variety of mutagenic agents have been 

 studied and foimd to parallel to a considerable degree their actions on 

 other living forms (Puck, 1957; Szybalski and Smith, 1959). The great 

 tendency of mammalian cells cultivated in vitro to become hyperploid 

 and aneuploid under certain conditions has been noted ( Moore et ah, 

 1956). The fact that this process is preventable by institution of very 

 careful monitoring of the physical and chemical environment has led to 

 the proposal that this chromosomal derangement is a result of a mitotic 

 inhibition occurring after the DNA has doubled, so that a polyploid 

 cell is produced which subsequently has an appreciable probability of 

 dividing unequally (as by a tri-polar mitosis) to produce aneuploid 

 progeny (Puck, 1960a). Study of the metabolic requirements" of the 

 diploid diploid vs. the aneuploid hyperploid forms has revealed the 

 latter to be much more self-sufficient nutritionally, and more resistant 

 to the action of inhibitory agents ( Fisher, Puck, and Sato, 1959; Puck, 

 Cieciura, and Fisher, 1957). Thus the greater selective advantage of 

 these aneuploid cells appears to explain their ability to overwhelm 

 tissue cultures in which they have once secured a foothold. It appears a 

 definite possibility that some tumors may owe their ability to overgrow 

 normal cells to this process. 



The chromosome number of normal man has now been definitely 

 ascertained to be 46, and the earlier claims for numbers like 47 and 48 

 appear to be unsubstantiated (Tjio and Levan, 1956; Ford and Ham- 

 merton, 1956; Tjio and Puck, 1958). In vitro techniques have made 

 possible grouping and identification of the human chromosomes (see 

 Figures 2 and 3). While some of the autosomes are closely similar, 

 small differences in arm length can be detected, provided that a suffi- 

 ciently large number of mitotic figures is available for examination and 

 these have been prepared with maximum control of the conditions so 

 as to insure the uniformity of state of the chromosomes. One of the most 

 interesting features of the human karyotype is the large difference in 

 size in the sex chromosomes, the X being one of the largest, while the Y 

 is close to the smallest. This is consistent with the relatively large num- 

 ber of sex-linked genetic defects in man, known to be carried on the X 

 chromosome. The large size-differential of these two chromosomes 

 results in the human female having 4 per cent more chromosomal ma- 

 terial than the male in each cell, a fact which has been suggested as a 

 possible explanation for the greater longevity of the human female 

 ( Tjio and Puck, 1958b ) . Moreover, X-containing sperm which yield fe- 

 male progeny would be more massive than those containing a Y chrom- 

 osome, and this differential effect could easily confer a greater motility 

 on Y-bearing sperm which could give them a slight but appreciable 

 competitive advantage over those containing an X chromosome. Hence 



