Ill PARTHENOGENESIS 91 



and fuse together. The egg behaves, indeed, as if it had been fertiUzed 

 by the second polar body, except that, since no halving of the chromosome 

 number has taken place in either nucleus, the resulting zygote nucleus 

 has double the normal diploid number, that is to say, 168 instead of 84. 



It should be noted that Brauer found this mode of meiosis much 

 rarer than the ordinary mode with only one maturation division, and that 

 Fries did not find any instance of it among his material. 



(2) Facultative Parthenogenesis 



The second type of parthenogenesis, where the egg matures in the 

 ordinary way and develops with the haploid number of chromosomes, 

 has been described in a few Hymenoptera. The classical example is 

 the honey-bee {Apis mellifica). It has long been known that the fate of 

 the bee's egg depends upon whether it is fertiHzed or not. The queen 

 bee is impregnated by the drone during the nuptial flight, and the sper- 

 matozoa are stored up in her receptaculum scminis. As the eggs pass 

 down the oviduct they pass the mouth of this receptacle, and according 

 as to whether a spermatozoon issues from it or not, the egg is or is not 

 fertilized. If fertiHzed, the egg develops into a female (either a queen or 

 a worker according to the food supphed to the larva) ; if not fertihzed 

 it develops into a male. 



The cytology of the bee has been thoroughly worked out by Meves 

 (1907) and Nachtsheim (1913). The diploid chromosome number is 

 thirty- two, though, as in many other animals {e.g. Ascaris, p. 81), this 

 number is greatly exceeded in the somatic tissues outside the germ- 

 track, and may reach as high a number as sixty-four. On the other hand, 

 in the oogonia the chromosomes tend to come together in pairs, giving 

 sixteen double chromosomes, much as in some Diptera ^ (p. 126). Thus 

 the determination of the chromosome number is fraught with some 

 difficulty, but it is revealed by the number in the gamete, which is 

 sixteen, and in the female embryo, where it is thirty-two. 



No difference is to be expected, nor as a matter of fact was observed, 

 between the meiotic processes of those eggs that are to be fertilized and 

 those that are not. After the second meiotic division the egg nucleus 

 leaves the surface of the egg (in which position, as usual, the maturation 

 divisions take place) and travels towards the centre. If the egg has been 

 fertiHzed it there meets with the male nucleus and an ordinary zygote 

 nucleus is formed. If it has not been fertiHzed, the egg nucleus continues 

 to travel right across the egg to the opposite side, as if in search of the 



1 A similar but less strong tendency to pair is observed in the spermatogonia (Nachtsheim) 

 which is particularly noteworthy, because these nuclei arc haploid. Moreover, the sixteen 

 double chromosomes in the oogonia unite into eight (tetravalent) chromosomes in the meiotic 

 prophase. See p. 92. 



