STRUCTURE AND FUNCTION IN MAMMALIAN FGGS 41 



most practicable during the prophase stages of the first cleavage 

 mitosis. A few possible examples of aneugamy have already been 

 recorded. Giant eggs undergoing fertilization and displaying a 

 single female pronucleus, which may well have been polyploid, 

 were recovered from rats (Austin and Braden, 1954c; see also p. 15). 

 Eggs from mated rats treated with colchicine had two normal- 

 looking pronuclei but no second polar body; the female pronuclei 

 seem likely to have been diploid (Austin and Braden, 1954b). Giant 

 spermatozoa are occasionally encountered (rat: R. Kinosita, i960, 

 personal communication; cat: M. W. H. Bishop and Austin, un- 

 published data); these are probably polyploid and could lead to 

 aneugamy if they are capable of fertilization. Dimegaly (two sizes) 

 and polymegaly (several sizes) of spermatozoa have long been known 

 in insects, nemertines, annelids, amphibians and birds; some forms 

 are considered to arise through suppression of one or both sperma- 

 tocyte divisions and would accordingly be polyploid (Wilson, 1928, 

 p. 303). 



Polyandry mid polygyny. Eggs recovered from treated as well as 

 from untreated animals at the time of fertilization have occasionally 

 been found to possess three well-formed nuclei. In some instances, 

 these were named as one female and two male pronuclei (rat : Austin 

 and Braden, 1953a, b; Austin, 1956b; Odor and Blandau, 1956; 

 Braden, 1958a; Piko, 1958 — mouse: Braden, Austin and David, 

 1954; Edwards and Sirlin, 1956; Braden, 1957; Edwards, 1957a— 

 hamster: Austin and Braden, 1956 — field vole: Austin, 1957b — pig: 

 Pitkjanen, 1955; Hancock, 1959, 1961; Thibault, 1959). In other 

 instances, pronuclei were identified as one male and two female (rat : 

 Austin and Braden, 1953b; Austin and Braden, 1954b, c — mouse: 

 Pesonen, 1949; Braden, 1957; Edwards, 1957a, b — rabbit: Thibault, 

 1949; Austin, 1960b— hamster: Hamilton and Samuel, 1956; Chang 

 and Fernandez-Cano, 1958; Ohnuki, 1959 — pig: Thibault, 1959). 

 In others again, identification was not made (rat: Tafani, 1889; 

 Ludwig, 1954 — mouse: Kremer, 1924 — cat: R. Van der Stricht, 

 191 1 ; Hill and Tribe, 1924 — ferret: Mainland, 1930 — rabbit: 

 Amoroso and Parkes, 1947; Austin and Braden, 1953b — pig: Pit- 

 kjanen, 1955 — cow: Pitkjanen and I vankov, 1956 — sheep: Pitkjanen, 

 1958). The presence of one female and two male pronuclei con- 

 stitutes the state of polyandry and arises from polyspermy — the 

 participation of two spermatozoa in fertilization. The reported 



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