8 



GENETICS 



the egg and sperm. The meaning of these things was at 

 first not known. But men made guesses about them; and 

 as time passed, some of the guesses were verified, so that 

 the significance of many of the things observed has been 

 discovered. 



What Things Are Found in the Germ Cells? — The germ 

 cells were early found to contain a complicated miniature 



Figure i. Chromosomes, in the condensed condition, in a dividing 

 cell of the salamander Amblystoma. 



apparatus, the nucleus. This nucleus was found to contain 

 a number of pieces of machinery, of a curious appearance, 

 which have been called the chromosomes (figure i). These 

 chromosomes are found too in all the cells of the body that 

 is later formed from the germ cells. Although they are 

 continually changing, growing, dividing, operating in vari- 

 ous ways, the chromosomes take on at regularly recurring 

 periods in the life of the cell certain clearly marked char- 

 acteristics. At these most conspicuous periods they appear 

 as rod-like, loop-like or granular structures. When the cell 

 divides, these structures are observed to go through a re- 

 markable and complex series of evolutions (figure 2). They 



