ix] SINGLE POLAR DIVISION 127 



modifications of the maturation divisions of partheno- 

 genetic eggs do occur, the purpose of which, if one may 

 speak thus ideologically, is to obviate this difficulty. The 

 maturation processes vary in different cases, and it will be 

 convenient to classify them under several heads. 



(i) Maturation with one polar division: no chromosome 

 reduction. 



This may be regarded as the most typical mode of 

 maturation of parthenogenetic eggs; it is found in the 

 parthenogenetic Crustacea (Phyllopoda, Ostracoda), in the 

 Aphids, in the paedogenetic ova of the Cecidomyid flies, and 

 probably in at least the female-producing eggs of Rotifers. 

 Although this type of maturation occurs in so many species, 

 the details have been worked out in comparatively few, 

 and there is room for much further investigation, before our 

 knowledge can be complete. It has long been known that 

 in members of the groups mentioned the parthenogenetic 

 eggs give off only one polar body, while the eggs that 

 require fertilisation produce two, and it is a matter for 

 surprise that more detailed work on the subject has not 

 been undertaken. One of the most complete papers on the 

 subject is that by MORGAN (1915) on Phyllaphis coweni and 

 other species (PI. XIII). In Phyllaphis the somatic number of 

 chromosomes is six. In the oogenesis of the fertilisable eggs 

 there is a typical contraction-phase (synizesis) from which 

 emerge three double chromosomes; the eggs undergo a typical 

 double maturation division, giving off two polar bodies, and 

 leaving three single chromosomes in the nucleus. In the 

 development of the parthenogenetic eggs, on the contrary, 

 there is no synizesis, six chromosomes are clearly visible in 

 the growing oocytes, and the egg undergoes only one polar 

 division, leaving six chromosomes in the mature egg-nucleus. 

 The occurrence of synizesis in the oocytes of parthenogenetic 



