9O E. L. SHAFFER. 



and Pelidonota no cross-shaped tetrads and only one ring tetrad 

 are found. The question of reduction division is difficult to 

 analyze here, with the exception of the ring tetrads. The latter 

 are always arranged on the spindle in the direction of the spindle 

 axis and the spindle fiber attachment is median. Consequently 

 the separation of the dyads occurrs at the point of the synaptic 

 union and the division is reductional. 



3. Sex Chromosomes. 



The earliest work on the sex chromosomes was done by Miss 

 Stevens ('05, '06) on the Coleoptera. She found the so-called 

 sex chromosomes in over forty species and her work and that 

 of Wilson's on the Hemiptera and McClung's on the Orthoptera 

 have been the basis of the later work correlating sex determina- 

 tion with the chromosomes. In the Coleoptera the sex chro- 

 mosomes are found as unpaired "accessory" and as unequal 

 elements which separate in one of the maturation divisions and 

 divide equationally in the other maturation division. Arnold 

 ('08) has maintained that in Hydrophilus piceus there are no 

 sex chromosomes. There is present in the growth stages a 

 chromatin nucleolus which may even persist up to the first 

 maturation division and may even pass undivided to one cell. 

 However, it disappears and cannot be found in any of the second 

 spermatocytes. 



In Ladmosterna, Pelidonota and Cotalpa the sex chromosomes 

 are of the xy type the y element being the smaller of the unequal 

 pair (Figs. 1-6). There are no marked differences in the size 

 and form of the sex chromosomes in the four species of Lach- 

 nosterna studied, but in Pelidonota the x element is considerably 

 larger than in the Lachnosterna material. In all cases the sex 

 pair separate in the first maturation division and divide equa- 

 tionally in the second, thus yielding two types of spermatozoa. 

 (Figs. 31, 32, 33, 35, 36). In a single case the sex chromosomes 

 failed to separate in the first maturation division, both chromo- 

 somes going into one of the daughter cells. This is undoubtedly 

 a case of non-disjunction similar to that which has been found 

 genetically and cytologically by Bridges ('16) in Drosophila. 



The sex chromosomes presist throughout the entire growth 



