28 MARGAKET MORRIS 



further reduction of chromosomes, the single spermatocyte 

 division being homot}T3ic, so that the haploid number appears 

 again in the spermatids. Doncaster ('06) found that in the par- 

 thenogenetic egg of the sawfly both polar bodies are formed 

 and the same is true of the eggs of the parasitic hymenoptera 

 investigated by Silvestri ('06, '08, '15). In all of these cases 

 the polar nuclei are retained in the cytoplasm of the egg, and 

 the second fuses with one or both of the daughter-nuclei result- 

 ing from a division of the first. The 'copulation nucleus' thus 

 formed does not, however, fuse with the egg nucleus. Some- 

 times it takes part in the formation of an envelope surrounding 

 the embryo — sometimes it degenerates. (See Silvestri.) 



In some other naturally parthenogenetic forms, however, 

 there is no reduction of the chromosomes of the egg before de- 

 velopnient begins. For instance, the eggs of the psedcgenetic 

 larva of Miastor undergo only one maturation division (Hegner, 

 '14 a). This is true also of the parthenogenetic eggs of Aphis 

 rosae and Aphis oenotherae, in which only one polar body is 

 formed regardless of the sex of the individuals developing from 

 the eggs (Stevens, '04). 



In some forms, the suppression of the true reducing division 

 is associated with the determination of the sex of the embryo 

 developing from the parthenogenetic egg, as Whitney ('08) has 

 shown to be the case with Hydatina senta. Here the females 

 come from eggs which have formed two polar bodies, and the 

 males from these which have formed but one. An essentially 

 similar case is that of the gallfly, in which some of the partheno- 

 genetic eggs form two polar bodies and the rest none at all. 

 The ones which have undergone the maturation divisions develop 

 into male flies with the haploid number of chromosomes; the 

 others have the diploid number and become females (Don- 

 caster, '10, '11). In the Phylloxerans (Morgan, '09) neither the 

 male-producing nor the female-producing parthenogenetic egg 

 forms two polar bodies, but the single maturation division re- 

 duces the number of chromosomes in one and not in the other. 

 Two accessory chromosomes are thrown out in the maturation 

 of the male-producing egg, while the female egg has the full 



