m'clung: chromosomes of anasa tristis. 373 



The position of the accessory chromosome upon the outside of 

 the group has led Foot and Strobell to coin another new name 

 for it, and in their terminology it is the "eccentric chromo- 

 some." 



4. What is the behavior of the accessory chromosome and of 

 the plasmasome in the first spermatocyte prophase ? 



While there is no dispute regarding the number of chromo- 

 somes in the first spermatocyte, there is serious divergence of 

 opinion with reference to the structure of the accessory chro- 

 mosome. Upon its nature depends the number of spermato- 

 gonia! chromosomes and the character of the four spermatids 

 derived from each first spermatocyte, so its determination is 

 most important. If it be a tetrad then the spermatogonia! 

 number must be even and the spermatids all alike in the pos- 

 session of eleven chromatids; but if, on the contrary, it be a 

 diad the spermatogonia! number of chromosomes is necessarily 

 odd and the spermatids of two types, one of which possesses 

 eleven chromosomes and the other ten. 



With the exception of Foot and Strobell all observers are 

 agreed that there are but two chromatids in the accessory 

 chromosome, but these observers claim that it possesses four 

 as do the other chromosomes. As proof for this contention, 

 they publish a series of chromosome groups from the late pro- 

 phase in which the various elements are clearly shown. The 

 identification of the accessory chromosome is correctly made in 

 each case, I believe, but it is entirely clear to my mind that 

 there is nothing in the appearance of these photographs to 

 justify the interpretation of the accessory chromosome as a 

 tetrad. There is in each case a characteristic difference be- 

 tween the tetrads and the accessory chromosome, for in the 

 former there always appears a diamond-shaped opening, the 

 angles of which point to the two lines of cleavage, while in the 

 case of the accessory chromosome this is lacking. Every ac- 

 cessory chromosome, on the contrary, shows a clear, straight 

 line of division along its entire length. In but a single instance 

 is there an exception, and that is in their figure 4, plate II, 

 where the plane of longitudinal cleavage is interrupted by 

 transverse markings. A careful examination of this chromo- 

 some will demonstrate, however, that there is not only one of 

 these apparent cross divisions, but two of them. I am con- 

 vinced that the excellent photomicrographs of Foot and Stro- 



