ix] PARTHENOGENESIS IN HYMENOPTERA 133 



persists and ultimately gives rise to the genital cells of the 

 larva, but this has been shown to be erroneous. Sometimes 

 all three groups of polar chromosomes unite into a single 

 mass, and SILVESTRI has shown that in certain parasitic 

 Hymenoptera of the familiesChalcididaeand Proctotrypidae 

 this united mass gives rise to a nucleus which later divides 

 and provides the nuclei for the envelope which encloses the 

 embryonic mass. With rare exceptions this only happens 

 in species which have embryonic fission; in those species 

 in which each egg gives rise to only one embryo, the polar 

 nuclei or chromosomes sooner or later degenerate completely 

 (cf. Fig. 10, p. 82). 



Development proceeds similarly whether the egg has 

 received a spermatozoon or not. If spermatozoa are present, 

 one of them conjugates with the egg nucleus, so giving rise 

 to the somatic (double) number of chromosomes. If the egg 

 is unfertilised, the egg nucleus segments normally, but, at 

 least in the earlier divisions, only the reduced number of 

 chromosomes is present. The embryo derived from a fer- 

 tilised egg develops into a female, that from an unfertilised 

 egg into a male. The male thus starts life with only half the 

 normal number of chromosomes, and this condition persists 

 throughout its life in the germ-cells and, at least in some 

 species, in the cells of the nervous system. In other tissues, 

 by a process which has not been sufficiently investigated, 

 the number becomes doubled, or in some cells even quad- 

 rupled, so that in most cells of the body the male and female 

 have the same number of chromosomes. 



Since the chromosomes are present in the reduced num- 

 ber in the spermatogonia before maturation, it is evident 

 that their number cannot be again halved in the spermato- 

 cyte divisions, and it is found in all the Hymenoptera in 

 which the spermatogenesis has been followed that one of 



