20 THE MAMMALIAN EGG 



microscopy, and there is no doubt that small structures of this kind 

 can sometimes be found by phase-contrast microscopy, but this 

 does not necessarily imply that they have migrated from the 

 nucleus or, indeed, that they are really forms of nucleoli. If migra- 

 tion does take place, it seems unlikely to involve a pinching-off 

 process, for this would surely have been seen in all its phases during 

 any of the more extensive investigations on mammalian eggs ; no 

 such records appear to have been made. It is possible, however, 

 that the nucleolus could pass through the nuclear membrane in a 

 physically divided state and reconstitute on the other side. Accor- 

 ding to Anderson (1953), substances with a molecular weight of 

 15,000 can traverse nuclear-membrane pores and evidence of actual 

 transfer of material through the pores into the cytoplasm has been 

 obtained by electron-microscopic observations on insect nurse-cells 

 (Anderson and Beams, 1956). Another possible mode of transfer, 

 and one that presumably would permit the passage of more highly 

 organized substances, is suggested by the finding of Gay (1956) of 

 minute but distinct outpocketings of the nuclear membrane which 

 she believes become detached and move into the cytoplasm. 



Anomalies involving oocytes include chiefly the presence of two 

 and sometimes more in a single follicle, the presence of two nuclei 

 and sometimes more in a single oocyte, and the occurrence of 

 'giant' oocytes. Polyovular follicles and multinuclear oocytes have 

 been described in a wide variety of mammalian species (Hartman, 

 1926, who reviews the earlier literature; Engle, 1927; Mainland, 

 1928; Evans and Swezy, 193 1; Ota, 1934; Dederer, 1934; Stockard, 

 1937; Lane, 1938; Pankratz, 1938; Waterman, 1943 ; Harrison, 1948; 

 Bacsich, 1949; Davis and Hall, 1950; Fekete, 1950; Dawson, 1951; 

 Skowron, 1956; Kent, 1959, i960). Both are common in the 

 opossum Didclphis and dog. Fekete found polyovular follicles at an 

 unusually high incidence (6.1 per ovary) in an inbred strain of 

 mouse (C58), and inferred that this showed an important influence 

 of heredity. Polyovular follicles are found more often in immature 

 ovaries and involving immature oocytes. Kent considers that the 

 incidence of both anomalies varies with oestrogen level. Informa- 

 tion on the ultimate fate of these anomalies is fragmentary. 

 O'Donoghue (1912) reported finding a mature polyovular follicle 

 in a specimen of Dasyurus and such a finding is rare; nevertheless, 

 Allen, Brambell and Mills (1947) and Fekete (1950) maintain that 

 at least some polyovular follicles must undergo ovulation and yield 



