78 PHYSICAL BASIS OF HEEEDITY 



For a count of only 100 cells the agreement with expecta- 

 tion is sufficiently close to show that independent assort- 

 ment takes place. 



In addition to the differences of attachment just exam- 

 ined there are other differences that Miss Carothers has 

 studied. A constriction is found in certain chromosomes 

 in some individuals (Fig. 32, No. 5) that is absent 

 in other individuals. In some individuals the tetrad is 

 separating so that the group looks like four beads in a line. 

 In other cases one member of the pair is not constricted, 

 while its mate is constricted. Similarly for another 

 chromosome. In one individual both halves of the dyad 

 show a constriction, while another individual has one 

 smooth and one constricted half. These same two kinds 

 of chromosomes also have in some cases terminal attach- 

 ment, and in other cases subterminal, making possible 

 further combinations that can be identified. 



Finally, there are two types of subterminal attach- 

 ment in two chromosomes of the series. In one type the 

 chromosome is bent further from the end than in the other. 

 Either of these two types may have a mate of the other 

 type with terminal attachment, thus giving several fur- 

 ther identifiable combinations. '^All possible combina- 

 tions of the dyads in these two types of heteromorphic 

 tetrads occur and segregate [assort] freely in relation 

 to sex.'' Miss Carothers points out that when three types 

 of the same chromosome exist ''we have a visible mechan- 

 ism whose behavior in the maturation divisions corre- 

 sponds to the segregation of triple allelomorphs.'' 



In addition to the 12 ordinary chromosomes certain 

 individuals may have a small thirteenth or even a four- 

 teenth chromosome. These are called supernumeraries. 

 In Circotettix they were found present in two of eleven 

 individuals examined. If present, it, or they, are constant 

 for all cells except that at the reduction division there may 

 be a new distribution. If one is present it may go to 

 either pole with reference to the sex-chromosome, and at 



