128 PARTHENOGENESIS [CH. 



forms seems to be variable. In the parthenogenetic Ostra- 

 coda (Crustacea), WOLTERECK (1898) and SCHLEIP (1908) 

 have described a typical synizesis in the development of par- 

 thenogenetic eggs, although the chromosomes emerge from it 

 in the somatic instead of in the reduced number. In Artemia^ 

 however, FRIES found no synizesis, although this stage is 

 represented in a typical manner in the nearly related 

 Branchipus, the eggs of which are fertilised, and KUHN 

 finds no definite synizesis in the summer eggs of Daphnids. 

 There appears, therefore, to be no general rule with regard 

 to the presence or absence of the synizesis stage in the 

 oogenesis of forms in which. the parthenogenetic eggs give 

 off only one polar body, but all^observers agree that these 

 eggs contain the somatic number of chromosomes, and that 

 the reduction division is absent. In those eggs which go 

 through the synizesis stage, either this is unconnected with 

 the pairing of the chromosomes, or, if pairing occurs, it is 

 of a temporary nature, and the paired chromosomes break 

 apart permanently as they do temporarily in Lepidosiren. 

 The very interesting problems connected with the differ- 

 ences between the male- and female-producing eggs in 

 animals having this type of parthenogenesis must be left 

 for consideration in a later chapter. 



A subdivision of the class of parthenogenesis described 

 above is that in which the eggs which will give rise to females 

 undergo no reduction division and produce one polar body, 

 while those which yield males have a typical reduction 

 division and two polar bodies. This type, confined, so far 

 as is known, to the Rotifers, will be referred to again in 

 the chapter on the cytological basis of sex-determination 

 (Chapter XI). 



(2) A second, and very rare, type of maturation process 

 in eggs which develop unfertilised was first described by 



