Mat 30, 1913] 



SCIENCE 



823 



scribed, and involving the breeding of 

 many thousands of these flies, have steadily 

 increased the probability that these car- 

 riers are nothing other than the chromo- 

 somes. These experiments make it almost 

 certain that in the cross we have been con- 

 sidering vfhite eyes and yellow color are 

 alike determined by the same chromosome, 

 while red eyes and yellow color must ob- 

 viously have been carried originally by dif- 

 ferent chromosomes, since they came from 

 different grandparents. 



There is here, as in the ease of the short- 

 winged flies, almost conclusive proof that a 

 single chromosome may be responsible for 

 the heredity of more than one character; 

 and experiments of the same type have 

 proved that a single chromosome may be 

 responsible for many characters — at least 

 twenty, and probably many more. Inde- 

 pendent microscopical investigation has 

 provided a very deflnite basis for this con- 

 clusion, having made it almost certain that 

 the chromosome is a compound body, which 

 includes many smaller elements, perhaps 

 different chemical substances, each of 

 which may play a definite part in deter- 

 mination. Both kinds of evidence indicate 

 that these different elements are arranged 

 in the chromosomes in linear series and in 

 a definite way. The chromosomes arise 

 from long threads, which split lengthwise 

 throughout their whole length during divi- 

 sion. In this way all the separate elements 

 or substances which they contain may be 

 equally divided and distributed to the 

 daughter cells. 



And this leads us finally to one more 

 point that now forms a center of interest 

 in these studies. Although (in such cases 

 as we have been considering) characters 

 that enter the hybrid together tend to come 

 out together in the grandchildren, they do 

 not always do so. As we have seen, in a 

 few of the grandchildren characters that 



were originally associated have separated 

 so as to produce new combinations — such, 

 for example, as the white-eyed gray flies, 

 or the red-eyed yellows. How can this be 

 reconciled with the conclusion that they 

 were originally borne by the same chromo- 

 some? A possible answer to this question 

 has been offered by Janssens's theory of 

 the " chiasmatype, " which has been more 

 specifically and very ingeniously worked out 

 by Morgan and some of his pupils. Refer- 

 ence has already been made to the fact 

 that at a certain period, shortly before the 

 germ-cells are formed, corresponding ma- 

 ternal and paternal chromosomes become 

 coupled in pairs, side by side (synapsis). 

 This process is always followed by a more 

 or less intimate union of the two threads, 

 perhaps in some cases by actual fusion. 

 The evidence is still more or less conflict- 

 ing as to exactly what follows; but it is 

 certain that at a later period two separate 

 and parallel threads again become distinct, 

 and these may separate so as to pass un- 

 changed into different germ-cells. These 

 two threads are believed by many observers 

 to be identical with those that originally 

 united in synapsis, but this is in dispute. 

 The fact of particular interest in this con- 

 nection is that the two threads often be- 

 come twisted around each other like the 

 strands of a rope; and the observations of 

 Janssens indicate that in some cases these 

 threads may fuse at certain points where 

 they cross and then split apart at these 

 points in the longitudinal plane. By this 

 process, as will be made clear by the ac- 

 companying diagram (Pig. 3), the possi- 

 bility is given of an orderly exchange of 

 certain regions of the threads between the 

 two chromosomes of each pair. Now, it 

 has been suggested that in this way two 

 chromosomes that originally carry (let us 

 say) AB and ab, may undergo such an 

 exchange as to produce the new chromo- 



