SUTTON 



plain that in the ordinary form having 

 from 24 to 36 chromosomes, the pos- 

 sibilities are immense. Table 1 below 

 shows the number of possible com- 

 binations in forms having from 2 to 36 

 chromosomes in the presynaptic cells. 



Thus if Bardeleben's estimate of six- 

 teen chromosomes for man (the lowest 

 estimate that has been made) be cor- 

 rect, each individual is capable of pro- 

 ducing 256 different kinds of germ- 

 products with reference to their 

 chromosome combinations, and the 

 numbers of combinations possible in 

 the offspring of a single pair is 256 X 

 256 or 65,536; while ToxopJieitstes, 

 with 36 chromosomes, has a possibility 

 of 262,144 and 68,719,476,736 differ- 

 ent combinations in the gametes of a 

 single individual and the zygotes of a 

 pair respectively. It is this possibility 

 of so great a number of combinations 

 of maternal and paternal chromosomes 

 in the gametes which serves to bring 

 the chromosome-theory into final rela- 

 tion with the known facts of heredity^; 

 for Mendel himself followed out the 

 actual combinations of two and three 

 distinctive characters and found them 

 to be inherited independently of one 

 another and to present a great variety 

 of combinations in the second genera- 

 tion. 



The constant size-differences ob- 

 served in the chromosomes of Brachy- 

 jtola early led me to the suspicion, 

 which, however, a study of spermato- 

 genesis alone could not confirm, that 

 the individual chromosomes of the re- 

 duced series play different roles in 

 development. The confirmation of this 

 surmise appeared later in the results 

 obtained by Boveri ^^ in a study of 



i** Boveri, Th., "Ueber Mehrpolige Alitosen 

 als Mittel zur Analyse des Zellkems," Verb, 

 d. Phys.-Med. Ges. zu Wurzbiirg, N. F., Bd. 

 XXXV., 1902. It appears from a personal 

 letter that Boveri had noted the correspond- 

 ence between chromosomic behavior as de- 



31 



larvae actually lacking in certain chro- 

 mosomes of the normal series, which 

 seem to leave no alternative to the 

 conclusion that the chromosomes 

 differ qualitatively and as individuals 

 represent distinct potentialities. Ac- 

 cepting this conclusion we should be 

 able to find an exact correspondence 

 between the behavior in inheritance of 

 any chromosome and that of the char- 

 acters associated with it in the organ- 

 ism. 



In regard to the characters, Mendel 

 found that, if a hybrid produced by 

 crossing two individuals differing in 

 a particular character be self-fertilized, 

 the offspring, in most cases, conform 

 to a perfectly definite rule as regards 

 the differential character. Represent- 

 ing the. character as seen in one of the 

 original parents by the letter A and 

 that of the other by a, then all the 

 offspring arising by self-fertilization of 

 the hybrid are represented from the 

 standpoint of the given character by 

 the formula AA : 2 Aa : aa.— that is, one 

 fourth receive only the character of 

 one of the original pure-bred parents, 

 one fouith only that of the other; 

 while one half the number receive the 

 characters of both original parents and 

 hence present the condition of the hy- 

 brid from which they sprang. 



We have not heretofore possessed 

 graphic formulae to express the com- 

 binations of chromosomes in similar 

 breeding experiments, but it is clear 

 from the data already given that such 

 formulae may now be constructed. The 

 reduced chromosome series in Brachy- 

 stola is made up of eleven members, no 

 two of which are exactly of the same 

 size. These I distinguished in my pre- 

 vious paper by the letters A, B, C, . . . 

 K. In the unreduced series there are 



ducible from his experiments and the results 

 on plant hybrids — as indicated also in foot- 

 note 1, /. c, p. 81. 



