^g TRANSACTIONS OF WAGNER 



MENDEL AND HIS LAW 



nucleus. The wall of the cell now begins to send a partition through the center 

 of the cell, thus breaking it into two cells, each with a nucleus of its own, just 

 like the nucleus in the other end of the cell, because its material has been so 

 exactly divided by this intricate process. Weismann suggests that when this 

 division occurs, the action is taken which determines that the daughter cell shall 

 inherit the qualities of the mother from which it is separated. He believes 

 that strung on this thread is a long series of what he calls determinants, each 

 of which decides some point in the development of the cell. Here, then, is the 

 mechanism of heredity. The reason why the offspring resembles the parent 

 is because the cell from which it sprang, the egg cell, had been detached from 

 the cell which produced the parent and shared its determinants. 



It is clear, however, that the ordinary animal shares the qualities of two 

 parents. If he were to have all the determinants from both parents, it is evi- 

 dent that he would have twice as many as either of them. In the next genera- 

 tion this number would be doubled again, until the animal's cells would be 

 thoroughly cluttered with determinants. It is an easily recognized fact that 

 a child resembles one parent in one respect and the other in another. The 

 reason for this Weismann has again interpreted to us. Just before an egg is 

 ready to be detached and sent out into the world its nucleus goes through what 

 is known as a reducing division. Instead of taking one half of each section of 

 the thread, the cell divides in two, one part taking one half of the threads, not 

 as before one half of each thread. By this process, the cell loses one half of 

 its possibihties of resembling the parent or the parental line from which it was 

 detached. This reducing division occurs both in the egg detached from the 

 body of the mother and also in the sperm cell as detached from the body of the 

 father. When now the egg is fertilized, that is to say, when the egg cell of the 

 one parent fuses with the sperm cell of the other parent, the number of de- 

 terminants rises once more to the full quota. The offspring has as many possi- 

 bihties as either parent, but half of its possibihties have been derived from one 

 parent and the other from a different parent. Accordingly, the new individual 

 will be a fresh combination of traits from both parents. By the time Weis- 

 mann had become entirely familiar with the facts of cell multiplication, he had 

 worked so closely that his eyes were seriously affected and from this time on 

 he could not do very much microscopic work. This, however, gave him abun- 

 dant time to think over the work he had already done, and it was this reflec- 

 tion quite as much as the preceding work which has made Weismann famous. 



