SEGREGATION OF HOMOLOGOUS CHROMOSOMES 451 



These expectations have been realized in part; and, further- 

 more, in two of these ten tetrads there is a third type with both 

 homologues having nonterminal fiber attachment, but with a- 

 secondary shifting so that one of the free arms is longer than the 

 other. One finds, besides such heteromorphic tetrads (plate 

 14, fig. 22c), homomorphic tetrads of the two expected types, 

 one with both arms long (plate 14, fig. 23a) and the other with 

 both arms short (plate 14, fig. 10a). 



Another peculiarity, that of a single constriction occurring 

 near the proximal ends (that is, the ends directed towards the 

 poles), may mark certain tetrads. An example may be seen 

 on plate 14, figure 21a. One dyad of such a tetrad (Chromo- 

 some number 3, plate 4) may be of three types; V-shaped (plate 

 5, fig. 31), plain rod (plate 5, fig. 32) or constricted rod (plate 

 5, figs. 31 and 32) ; these occur in various combinations, fur- 

 nishing a visible mechanism for possible triple allelomorphs. 



4- Methods and ack nowledg?nents 



The material for the somatic mitoses of the female was fixed 

 in a modification of Bourn's solution which has been developed 

 in this laboratory and which promises to be very useful. The 

 following is the formula : 



Picric acid, saturate aqueous solution 75 cc. 



Formalin (full strength) 15 cc. 



Glacial acetic acid 10 cc. 



Urea, crystals •. . . . £ gm. 



After immersion in this fluid, material may be removed any 

 time between twenty-four hours and three months. It should 

 then be washed in water some fifteen minutes and run up gradu- 

 ally to 75 per cent alcohol where it may be kept indefinitely. 

 The fact that specimens may be left for a long time in this fixa- 

 tive makes it especially desirable for field work and for those not 

 equipped to make use of more elaborate methods. Acrididian 

 metaphase chromosomes are as well preserved as in Flemming. 



The testes, were fixed in strong Flemming solution. Sections 

 were cut from 7 to 12 microns in thickness; 10 proved best 

 suited to this work. The stains used were Flemming's tricolor 

 and Heidenhain's iron-hematoxylin. 



