The Chromosomes in Diabrotica 4^7 



and separate for the second division without the formation of a 

 nuclear membrane. The second spermatocyte equatorial plates 

 are shown in Figs. 26 and 27, the heterochromosome (x) appearing 

 in Fig. 26. All of the chromosomes divide in this division, giving, 

 as usual, two equal classes of dimorphic spermatozoa. The 

 spermatids and spermatozoa are similar to those of Diabrotica 

 vittata. The chromatin nucleolus is found in the earher stages 

 but is not visible in stages corresponding to Figs. 14 and 15, and 

 the head of the mature spermatozoon is only about one-half as 

 long. 



Type Ila 



About two-thirds (33 out of 100 males collected at Mountain 

 View, Cal.) of the individuals belonging to the second type 

 have one additional small chromosome, making twenty in the 

 spermatogonia (PI. II, Fig. 28). The additional chromosome 

 appears as a second heterochromosome in the growth stages (Fig. 

 29, s). In the first spermatocyte spindle the larger heterochro- 

 mosome (x) is found, as usual, near one pole of the spindle, while 

 the smaller one {s) may be in the equatorial plate (Fig. 30) or on 

 either side of it (Figs. 31, 32, 33), closely associated with x or as 

 widely separated from it as possible (Figs. 33 and 31). Fig. 34 

 is a polar view with the two heterochromosomes near one pole of 

 the spindle. The small chromosome may or may not divide in 

 the first division. In some individuals it almost always (possibly 

 always) divides as in Fig. 35 later than the other chromosomes. 

 In other cases it may be found undivided between the daughter 

 plates (Fig. 36), outside of one of them (Fig. 37), or it may be 

 concealed in the general polar mass of chromatin. In the telo- 

 phase and brief rest stage (Figs. 38 and 39) it is often quite dis- 

 tinct from the remainder of the chromatin. Whether it divides 

 in this mitosis or goes undivided to one pole or the other seems to 

 be a matter of chance, depending perhaps on the part of the spindle 

 which it happens to enter in the prophase. It seems to be much 

 less automatic in its behavior than the other chromosomes. This 

 peculiarly erratic behavior of the small heterochromosome in the 



