4G6 E. ELEANOR CAROTHERS 



these vesicles as expanded conditions of certain granules, ter- 

 minal in the case of his tetrads 'A' and 'B'. We also know from 

 from the work of Pinney ('09) that the point of fiber attachment 

 is usually marked by prominent granules. It, therefore, seems 

 probable, especially since the other end lacks them, that the vesicles 

 mark the point of fiber attachment on what should have been a 

 J-shaped tetrad and that their formation has weakened the dyad 

 with which they are associated to such an extent that division 

 in metaphase occurs at this point. The result is that one and a 

 half dyads go to one pole, the remaining half dyad to the other. 

 This peculiarity seems to be constant for this animal. Figure 

 38a (plate 11) represents a similar possible origin for an unequal 

 pair in one of the larger chromosomes in form B. This was the 

 only instance of the kind found in this animal. 



e. Other forms of heteromorphism. Two other types of hetero- 

 morphism remain to be considered. The first — a peculiarity 

 common to both these genera — is a constriction of certain dyads 

 of chromosomes number 3 and number 5. The behavior of tetrad 

 number 5 in different individuals is shown in plate 14. Figures 

 21a and 13a are homomorphic, constricted and smooth forms, 

 respectively. Figure 62/ is the heteromorphic form. The same 

 thing is shown in chromosome number 3 (plate 2) , animal number 

 6 being homomorphic for the constricted form, number 2 homo- 

 morphic for the smooth rod and number 8 heteromorphic. Tet- 

 rads marked by this peculiarity are almost as striking as the 

 J-shaped tetrads. And since chromosome number 5, frequent- 

 ly, and number 3, occasionally, are J-shaped or even of the 

 Stenobothrus type, the J-shaped tetrads may have the telomitic 

 dyads of the constricted form (plate 3, 16, chromosome no. 3); 

 or of the usual smooth rod form. In other words, for these two 

 chromosomes, there are three types of homologues: atelomitic, 

 telomitic-smooth and telomitic-constricted. 



The third type of heteromorphism affects chromosomes num- 

 ber 9 and number 1 1 as they are arranged on the plates. Both 

 homologues are atelomitic, but there has been a secondary shift- 

 ing of the point of fiber attachment so that the free arm of one 

 dyad is much longer than the free arm of the other. The point 



