MICHIGAN ACADEMY OF SCIENCE. 53 



THE HORMONE THEORY OF CHROMOSOME ACTION. 



BY ERNST A. BESSEY. 



That the i)henomena of heredity are bound up intimately with 

 those nuclear structures called chromosomes is the conclusion of 

 almost all students of the subject. The reasons for this belief may 

 be briefly reviewed. In ordinary nuclear division (mitosis), we 

 find at one stage a continuous or interrupted thread (the spirem) 

 becoming segmented into a definite number of pieces, the chromo- 

 somes. In plants all the chromosomes in a given nucleus are 

 usually very similar to one another, but in many animals, e. g., 

 some insects, they differ markedly. In the further course of 

 mitosis, these chromosomes split longitudinally and the resulting 

 halves are drawn to the opposite poles of the nucleus where each 

 regenerates its missing half and they finally assume their places 

 in the daughter nuclei. At the next nuclear division the same num- 

 ber of chromosomes appear and they will be found to have the 

 same shapes and location as in tlie previous division. In fact this 

 phenomenon is so widespread that biologists are now very strongly 

 inclined to believe in the confinued individuality of the chromo- 

 somes from one nuclear generation to the next. 



Still more marked is the peculiarity of the behavior of the 

 chromosomes in reproduction. Both the male and the female sex 

 cells (gametes) are found to have the same number of chromo- 

 somes,* and on careful comparison these are found to match in the 

 two cells. The zygote nucleus produced by the union of the two 

 gamete nuclei has, then, a double set of chromosomes; i. e., two 

 of ever}^ kind present in either of the gametes. Before the next 

 sex cells are produced, there occurs that peculiar process called 

 the reduction division or meiosis, in which the number of chromo- 

 somes is reduced again to the number found in the previous 

 gametes. Careful investigation has shown that in this process one 

 of each pair of chromosomes (which one appears to be a matter of 

 chance) passes entire to one daughter nucleus while the other 

 chromosome of the same pair goes to the other daughter nucleus. 

 This process differs from ordinary mitosis among other things in 

 that whole chromosomes not merely halves of the same chromo- 



*I have left out of consideration here the sex determining chromosomes. 



