458 E. ELEANOR CAROTHERS 



vides a mechanism for carrying these laws into effect that the 

 number of scientific people who question the proposition that 

 the chromosomes are the bearers of the hereditary factors has 

 become almost negligible. 



Mendel's first law in substance is, that of a pair of contrasting 

 unit factors contributed by the male and female parents, 

 respectively, one will dominate the other in the first generation, 

 but that during the gametogenesis of these individuals each 

 member of a pair of factors segregates into a different germ cell. 

 This is known as the law of segregation. 



Mendel's second law is that when a number of pairs of 

 unit factors are present the pairs assort independently of each 

 other. That is, while the first law describes the behavior of the 

 individuals of a pair, the second applies to the relation of the 

 pairs to each other. This is sometimes called the law of inde- 

 pendent assortment of different pairs of unit factors. 



Mendel, of course, was dealing with unit characters from a 

 purely genetical standpoint. Shortly afterwards, the work of the 

 early cytologists led to the Roux-Weismann theory of heredity, 

 which recognizes the chromosomes as the bearers of the heredi- 

 tary factors. Van Beneden in 1883 reported that egg and sperm 

 contribute equal amounts of chromatin to the new individual 

 in Ascaris megalocephala. Sutton in 1902 first clearly showed 

 that the chromosomes occur in a duplex size series and pointed 

 out that the behavior of the chromosomes during maturation 

 and fertilization is such as would be necessary for carrying out 

 the laws of heredity discovered by Mendel.^ 



For a demonstration, however, two things were necessary: 

 First, to find some species in which homologous chromosomes 

 (those which unite in synapsis) differ from each other and from 

 the remainder of the complex in such a way that they can be 

 certainly identified. Second, to be able to breed the organisms 

 freely in captivity in order that the behavior of the unlike homo- 

 logues might be studied in both parents and offspring. 



1 This paper is intended to be taken in connection with the one published in 

 1917, deciding as it does questions left open at that time. For this reason it is 

 not as complete in itself as it might otherwise be desirable to make it. 



