THE BIOLOGICAL BACKGROUND OF GENETICS 



20 1 



generation gone through a period of specialization for any particular 

 bodily function. In animals these cells are believed to be set apart 

 early in ontogeny (the development of the individual) and localized 

 in sex glands, or gonads. In plants the germ track is believed to be 

 maintained in the meristematic cells of growing points, but germ cells 

 are not so definitely localized as in most animals. 



Fig. 44. — The germ track in Ascaris. Stages in early cleavage showing the 

 chromatin diminution process in all cells except the stem cell (S). (From Boveri, 



lSQ2.) 



In some animals the germ track is very clear and unmistakable 

 from the beginning of one generation to that of the next. The classic 

 instances of germinal continuity are those of the roundworm, Ascaris 

 megalocephala, and the fly, Miastor americana. 



In the variety of Ascaris known as univalcns, which possesses only 

 two chromosomes to each cell (incidentally the lowest number known), 

 the developmental stages are as follows: at the first division of the 

 zygote two cells are formed in the usual way, each cell with two long 

 loop-like chromosomes (Fig. 44, A). One of these two cells, however, 

 undergoes a striking nuclear change involving the breaking-off of the 



