cytology: the study of the cell 143 



than the latter organisms. The quantity of nucleic acid present in a set of 

 chromosomes, therefore, is probably not a measure of the number of genes 

 present. We are not justified in exactly equating genes with nucleic acid. 

 It is likely that genes are not pure nucleic acid but a combination of nucleic 

 acid and protein, the nucleic acid imparting to the gene its specificity. The 

 chromosomes may also contain nucleic acid which is unbound to protein. 

 Only when properly combined can nucleic acid and protein take on genie 

 properties. 



Recent work by McClintock (1951), Lewis (1951), and others suggests 

 that the gene may be in reality a compound structure, its parts separable un- 

 der certain conditions. It has also been found that the action of a gene may 

 depend to some extent on its position in the chromosome. There are in most 

 organisms so far studied certain regions of the chromosomes which seem 

 wholly or almost devoid of definitive genes. These areas, known as hetero- 

 chromatic regions, are imperfectly understood as to their structure and func- 

 tion. It has been found, however, that when a gene whose normal position is 

 distant from heterochromatin is transferred to a heterochromatic region, its 

 behavior may become fluctuating — it may function in some cells and not in 

 others, and a mottled or mosaic effect may be produced. This kind of "posi- 

 tion effect" is especially well known in the fruit fly, but Catcheside (1947) 

 has found a very striking case in Oenothera, the evening primrose, and a few 

 other cases are known. The chemical basis for this behavior is not under- 

 stood. 



From this brief account it is clear that much progress has been made in 

 the attempt to understand the nature of the gene and how it functions. We 

 know where genes are in the cell, and we can follow many of them when they 

 become transferred from one place to another, or when they change their 

 function. We have learned some facts regarding the chemical structure of 

 the chromosomes of which they are a part. We have a long way to go, how- 

 ever, before our knowledge of genes becomes in any way complete or ade- 

 quate. This is one of the main emphases of the cytologist at the present time, 

 and in this quest he must use a great variety of technical approaches in 

 addition to those of microscopy, or else collaborate with those who are 

 capable of using these techniques. 



Another focus of interest which is occupying the attention of many cytol- 

 ogists at the present time is the cytoplasm — that portion of the cell which 

 lies outside of, and surrounds, the nucleus with its chromosomes and genes. 

 For a long time cytologists tended to place relatively little emphasis on the 

 cytoplasm, largely because there seemed to be so little that could be ob- 

 served microscopically in this portion of the cell. Bodies as striking as chro- 

 mosomes were seldom present; very few definitive activities seemed to ac- 

 company cell division, in contrast with nuclear division. Some bodies, to be 

 sure, were to be seen, but except for plastids (chloroplasts, etc.) none of 



