SEX DETERMINATION AND SEX DIFFERENTIATION 225 



whole rather intricate story is thus seen to be merely a variant upon 

 the typical scheme of chromosomal sex determination. 



b) Haploid parthenogenesis. — This kind of parthenogenesis is now 

 known to occur in rotifers, in several orders of insects, and in arach- 

 nids. It is practically universal among the Hymenoptera (bees, wasps, 

 ants, etc.), and we may use the case of the honey bee as an illustration. 

 In haploid parthenogenesis the egg develops after having undergone the 

 reduction division ; it therefore has only half the somatic number of chro- 

 mosomes, including but one X-chromosome. Invariably the progeny 

 from haploid parthenogenesis are males, which we might expect from 

 the fact that they have but one X-chromosome. In the bees the queen 

 seems to be able to determine whether an egg gets fertilized or not. 

 An egg descends the oviduct, passes the seminal receptacle containing 

 a supply of sperms acquired during the mating act, and if sperms are 

 given off, fertilization occurs and a female is produced; but if an egg 

 slips past the seminal receptacle without being fertilized, the result is 

 a male (drone). Now these drones are the mates of the future queens, 

 and must supply the spermatozoa for the next generation of eggs. They 

 already possess the reduced number of chromosomes, so they cannot 

 well undergo the reduction division in forming gametes. It is inter- 

 esting to note, however, that a sort of vestigial reduction division takes 

 place resulting in the formation of a tiny cell without any nucleus and 

 a larger cell with all the chromosomes (including one X-chromosome) 

 characteristic of males of the species. A second maturat ion division di- 

 vides chromosomes lengthwise. Since all gametes, both male and fe- 

 male, contain an X-chromosome, fertilization always results in a female. 

 Thus once more the general sex-determination formula is confirmed. 



Sex-chromosomes in hermaphrodites and gynandromorphs. — 

 Hermaphrodites are individuals which are functionally both male and 

 female, that produce both eggs and sperms in the same body. Her- 

 maphroditism is common in snails, flatworms, earthworms, nematodes, 

 tunicates, and in several other phyla of animals. We have unfortu- 

 nately very little information about the chromosome situations in 

 these forms. In one species of nematode {Angiostomum nigrovenosum) , 

 however, it is known that there is an alternation of generations between 

 a parasitic hermaphroditic generation and a free-living dioecious gen- 

 eration (with separate males and females). In the dioecious genera- 

 tion males and females are about equally numerous. All fertilized 

 eggs of this generation produce parasitic hermaphrodites. These pro- 

 duce from their gonads first oogonia and later spermatogonia, the form- 



