REPRODUCTION AND HEREDITY 139 



to be strangely connected with the determination of 

 sex. It appears at first sight anomalous that in an 

 ordinary community of hive-bees all the male members 

 (" drones ") should be without any inherited characters 

 derived through a male parent, but as the queen-bee 

 develops from a fertilised egg, each drone has a maternal 

 grandfather. 



In many of these cases the facts of virgin-reproduction 

 have been proved to correspond with some abnormal mode 

 of nuclear division among the germ-cells. Thus in Aphids 

 and their allies the Phylloxerans, T. H. Morgan has shown 

 (1909) that the eggs of the parthenogenetic females 

 mature without reduction ; only one polar body is extruded 

 and the number of chromosomes remains at the full 

 " diploid " complement (zn). In the sexual broods of 

 aphids which produce the winter eggs, while the females 

 have the full double number (zn) the males have one or 

 two fewer (zn— 1 or 2^2 — 2). This is brought about by a 

 partial reduction during the maturation of the male- 

 producing eggs of the virgin females of the last generation, 

 one or two chromosomes passing undivided into the polar- 

 nucleus, so that the ripe- egg nucleus has one or two fewer 

 than the full double number. It has been mentioned that 

 the fertilised winter eggs, which the sexual aphids produce, 

 all develop into female insects, when hatched the next 

 spring. In the spermatogenesis of the autumn males the 

 usual reduction-division takes place, but only those sperma- 

 tocytes whose nuclei contain the full single (" haploid ") 

 number of chromosomes (n) develop so as to give rise to 

 active spermatozoa ; all those without the ;c-chromosome 

 (« — I or « — 2) are much below the normal size and cannot 

 produce functional sperm- cells. Hence it follows that 

 every fertilised egg has the full diploid number of chromo- 

 somes and develops into a female insect. 



Among the Hymenoptera, as has already been mentioned, 

 the number of chromosomes in the cell-nuclei of a male 

 insect is half the number that characterises the female^s 

 nuclei ; in bees, for example, a male's nucleus has sixteen 



