38 THE MAMMALIAN EGG 



these had developed with or without second-polar-body expulsion. 



Cold-shock treatment (hypothermia) had no significant effect 

 upon the incidence with which unpenetrated hamster eggs under- 

 went activation or developed nuclei (Austin, 1956a). In rabbits, 

 sheep, rats and mice, however, the incidence is greatly increased by 

 cold shock as well as other forms of experimental stimuli. In most 

 rabbit eggs chilled in situ by the application of ice to the Fallopian 

 tube, single diploid nuclei were formed, the second meiotic divi- 

 sion being suppressed (Thibault, 1947, 1948, 1949; Chang, 1952a); 

 sheep eggs seemed to react in the same way (Thibault, 1949 ; Thibault 

 and Ortavant, 1949). In rabbits, other procedures were also effec- 

 tive: culture in vitro, or treatment with heat (47°C), with hypertonic 

 solutions or with suspensions of spermatozoa (Pincus, 1936b, 1939a), 

 hypothermia (Shapiro, 1942). (It has been claimed that partheno- 

 genesis in the rabbit can proceed to the birth of viable young: 

 Pincus, 1939a, c; Pincus and Shapiro, 1940a, b.) In rats, chilling 

 caused about 10 per cent of the eggs to show nucleus formation 

 and on the evidence available all these eggs could be held to have 

 completed the second meiotic division so that the nuclei were 

 probably haploid (Austin and Braden, i954d). In mice, the same 

 result, though at a higher incidence (about 40 per cent), required 

 a different treatment, namely heat shock (immersion of the Fallopian 

 tubes in water at 44 to 45 °C); a few eggs of the same kind were 

 recovered when the treatment had been merely ether anaesthesia 

 (Braden and Austin, 1954c). 



There is no certain evidence that mammalian eggs developing 

 single nuclei, whether haploid or diploid, can give rise to embryos 

 capable of surviving to birth, but some embryonic development is 

 known to be possible — to 2- and 4-cell eggs in the sheep and rodents, 

 and to blastocysts in the rabbit (one of which implanted — Thibault, 

 1949). The nuclei themselves, however, have a definite interest. In 

 rats and mice, these nuclei were found to be capable of achieving 

 roughly twice the nuclear and nucleolar volumes of normal female 

 pronuclei, despite the fact that they derived from equivalent 

 chromosomal material; the possible significance of this observation 

 is discussed later (p. 47). Beatty (1954) has recorded the finding of 

 spontaneous haploid mouse embryos which had reached the blasto- 

 cyst stage of development (3 \ days) ; they may have arisen partheno- 

 genetically, but since they came from mated animals origins through 

 androgenesis or gynogenesis cannot be excluded. 



