PROBLEM I. Why Offspring Resemble Their Pare?its 



457 



Fig. 405 Stages in nuclear 

 division {mitosis) in plant 

 cells. In (a) the nucleus is 

 not dividing, hi (b) the 

 threads of chromatin have 

 appeared; chromosonies are 

 fully formed in (c). In (d) 

 the chromosomes are lined 

 up in the middle of the cell. 

 They have already split 

 lengthwise. The halves have 

 separated in (e). In (f) the 

 two new ?iuclei have begim 

 to for?n. The dark line in 

 the fniddle is the begijining 

 of the new cell wall, (a, b — 



KLINE; C, d, e, f — GENERAL 

 BIOLOGICAL SUPPLY HOUSE) 



rhroi}ioso777es (crow'mo-soams). There 

 is evidence that the genes are arranged 

 in a single row within the chromosomes 

 Hke a string of beads. The chromosomes 

 are jellyhke in composition, yet so firm 

 that they can be pushed around with a 

 microneedle. They are in pairs. Each 

 pair differs somewhat from the other 

 pairs in size and shape. 



While the chromosomes are forming, 

 the nuclear membrane disappears and the 

 rest of the nuclear material becomes 



mixed with the cytoplasm. The chro- 

 mosomes then move to the center of the 

 cell. At this time or even earlier the chro- 

 matin makes more of itself, with the re- 

 sult that there is double the original 

 amount. It is believed that in this process 

 each gene reproduces itself. Thus there 

 comes to be a double string of genes in 

 each chromosome. Each chromosome 

 then splits in two along its length from 

 end to end. Each of the two parts gets a 

 complete string of genes. When the 



