APOGAMY, APOSPORY, AND PARTHENOGENESIS 319 



develop into workers or into queens with the diploid number (32) of 

 chromosomes; those not fertilized develop into drones with the haploid 

 number (16). (At the time of spermatogenesis in the drone no further 

 reduction in chromosome number occurs: the spermatozoa retain the 

 number present in the body cells (16).) 



In the gall-fly, Neuroterus lenticularis, Doncaster (1910-1911) has 

 shown that there are two classes of parthenogenetic females. The egg of 

 the first class gives off no polar bodies, retains the diploid number (20) 

 of chromosomes, and develops parthenogenetically into a sexual female. 

 The egg of the second class gives off two polar bodies, retains the reduced 

 number (10) of chromosomes, and develops parthenogenetically into 

 a male. (The offspring of the sexual females and males constitute the 

 next generation of parthenogenetic females.) 



There are thus several organisms in which both zygogenetic and 

 parthenogenetic eggs are produced. In some of them, such as the bee, 

 in which the same egg can develop in either way, the two classes of eggs 

 show no morphological differences. In other forms, such as a species of 

 Melanoxanthus (a plant louse) and Sida crystallina (crustacean), they 

 may differ considerably. The parthenogenetic egg, for example, may 

 contain much less yolk than the zygogenetic one : it is less highly differen- 

 tiated, and "still retains the capacity to initiate dedifferentiation and 

 reconstitution independently of union with a male gamete. In this 

 respect it resembles the less highly specialized cells of other tissues 

 rather than the gametes" (Child 1915, p. 408). 



It has recently been shown that frogs which have been induced to 

 develop parthenogenetically from punctured eggs (Bataillon's method) are 

 of both sexes (Loeb 1921). The chromosome number in the females has 

 not been determined, but both Parmenter (1920) and Goldschmidt (1920) 

 report the diploid number in males so derived. The origin of this diploid 

 condition has not been satisfactorily explained. Parmenter suggests 

 that it may be due to the retention of one polar body, or to a premature 

 division of the chromosomes without cytokinesis just before the first 

 cleavage. This promises to be an interesting case in connection with 

 the mechanism of sex-determination. 



Conclusion. To review the various theories which have been advanced 

 to account for the origin of parthenogenesis, its relation to other forms of 

 reproduction, and its significance in the life history, is a task which lies be- 

 yond the scope of the present work: it has been our purpose only to indicate 

 some of the outstanding cytological facts in certain conspicuous instances 

 of the phenomenon. The cytological features have been accurately 

 ascertained in only a very few cases, and these show little agreement. 

 Furthermore, it is in artificially induced rather than in natural partheno- 

 genesis that the physiological conditions are best known. In view of 

 these facts it appears more than probable that many more cytological 



