THE GREAT RELAY RACE 



163 



that after half the contents of the nucleus has been disposed of there remains 

 four chromosomes; that within each chomosome there are, beyond the 

 range of vision, hundreds of genes ; and that it is possible within a single 

 chromosome to determine not only the relative arrangement of the many 

 genes, but also to find out the relative distance between any two of these 

 genes, it wUl be realized that the analysis of the germplasm has gone a long 

 way." ' 



The Role of the Cytoplasm 



In spite of the demonstrated importance of the chromosomes and 

 their genes in the mechanism of heredity, they are not the whole story. 

 There is the cytoplasm to be reckoned wdth, particularly in the egg. 

 In no cell can either the nucleus or the cytoplasm lead an independent 

 existence. Each depends upon the other. Hence, while the un- 

 doubted significance of the chromosomes is being emphasized, it is 

 well to remember also the indispensable cytoplasm. Is there such 

 a thing as cytoplasmic inheritance, in addition to that of the genes ? 



In answer to this question it is necessary in the first place to dis- 

 tinguish the part that cytoplasm plays in development as well as its 

 possible function in hereditary transmission. 



The nuclear membrane separates the chromosomes from the sur- 

 rounding cytoplasm during the resting stage of every cell cycle, 

 resulting in some degree of temporary independence. However, 

 every time mitosis is repeated this protective membrane vanishes for 

 the time being, leaving the chromosomes directly exposed to the 

 cytoplasm. Here, then, is furnished an opportunity for exchange of 

 materials between chromosomes and cytoplasm, and this exchange 

 does undoubtedly occur. During mitosis, it will be remembered, 

 each chromosome splits lengthwise, and the half chromosomes thus 

 formed, mingling freely with the cytoplasm, migrate to their respec- 

 tive poles. Meanwhile they are restored to their original dimensions 

 by the intake of material from the cytoplasm itself. Thus a part 

 of the cytoplasm of the cell becomes made over into chromosomal 

 material. 



In the long series of successful mitoses by means of which the 

 zygote eventually becomes an adult individual, the chromo.somes in 

 each newly formed cell still maintain their original genetic make-up as 

 to form and numbers of pairs. The cytoplasm of these various cells, 

 on the other hand, undergoes transformation to constitute the different 



' From W'alter, H. E., Genetics. By permission of The Macmillan Cominiiiy, publishers. 



