CHROMOSOMES AND MITOCHONDRIA 265 



and the drones parthenogenetically. The history of 

 the chromosomes has here been worked out by 

 Nachtsheim (1913). The primary oocyte contains 

 sixteen chromosomes in the form of eight tetrads ; the 

 mature egg and polar bodies are each provided with 

 eight chromosomes (Fig. 71, E) ; the inner half of the 

 divided first polar body fuses with the second polar 

 body, forming a " Richtungskopulationskern " (Fig. 

 71, F) which does not give rise to the male germ cells 

 as Petrunkewitsch (1901) claimed, but degenerates. 



The cleavage nucleus in the parthenogenetic egg 

 which produces the male shows sixteen chromosomes 

 which divide to form thirty-two or sixty-four in 

 the somatic cells, but do not increase in number in 

 the spermatogonia. The first maturation division 

 is unequal, and a "polar body" without any chroma- 

 tin is pinched off (Fig. 71, A-C, Rh). The sperma- 

 tids are likewise of two sorts ; the smaller (Fig. 71, 

 C, Rkz) contain as many chromosomes as the larger 

 (16), but degenerate, while the larger transform into 

 spermatozoa. The fertilized (female) eggs possess 

 the same number of chromosomes as the partheno- 

 genetic eggs, plus an equal number which is brought 

 in by the spermatozoon. The cleavage nucleus 

 exhibits thirty-two chromosomes which may become 

 sixty-four in the somatic cells, but unite two by 

 two to form sixteen in the oogonia. 



Phylloxera carycecaulis will serve to illustrate 

 the chromosome cycle in a species with a life cycle 

 composed of parthenogenetic females which alter- 

 nate with sexual males and females (Morgan, 1909, 



