110 THE MAMMALIAN EGG 



Donker (1955), Henriet (1955), Dziuk, Donker, Nichols and 

 Peterson (1958), Hafez (1958) and Noyes and Dickmann (i960). 

 The original reports are summarized in Appendix No. 1. 

 It has been demonstrated that : 



(a) Normal young animals can be born from embryos transferred 

 during the early cleavage stages ; this has been shown in the rabbit, 

 mouse, rat, sheep, cow and pig. 



(b) Follicular or tubal oocytes can undergo fertilization after 

 transfer to a mated recipient animal and develop to normal birth; 

 this has been shown in the rabbit, mouse, rat and sheep. 



(c) Eggs and cleavage embryos can tolerate wide variations in 

 environmental conditions between recovery from the donor and 

 lodgement in the host. Rabbit oocytes have survived storage at 

 o°C for 72 hr and at io°C for 96 hr, and rabbit embryos storage at 

 o°C for 78 to 102 hr or at io°C for 80 to 101 hr (Chang, 1947, 

 1948a, b, c, 1952a). Unfertilized rabbit eggs and fertilized eggs in 

 various stages of cleavage have been subjected in vitro to irradiation 

 from radiocobalt, and then transferred to suitable recipients (Chang, 

 Hunt and Romanoff, 1958; Chang and Hunt, i960). Even 65,000 r 

 did not prevent unfertilized eggs undergoing fertilization after 

 transfer, though subsequent development failed; most eggs, how- 

 ever, whether unfertilized or cleaving, were prevented by treatment 

 with 100 or 200 r from advancing far in embryonic development. 

 Apparently normal young rabbits and mice have been born from 

 2-cell eggs in which one blastomere was destroyed (Seidel, 1952; 

 Tarkowski, 1959a, b), and some embryonic development was 

 possible even from 4-cell eggs in which three blastomeres were 

 destroyed (Seidel, 1956, i960; Tarkowski, 1959a, b). Rabbit and 

 mouse embryos have been grown in culture for 1 or 2 days and 

 then, on being transferred to recipients, have developed to birth 

 (Chang, 1948c, 1950b; Biggers and McLaren, 1958; McLaren and 

 Biggers, 1958). Mouse oocytes have retained their capacity for 

 fertilization and extensive development after being frozen for \ to 

 31 hr (Sherman and Lin, 1958, 1959). Sheep embryos have with- 

 stood transfer to the rabbit genital tract for a week and then, after 

 retransfer to the uterus of a sheep, have developed for a further 10 

 to 12 days (Averill, Adams and Rowson, 1955; Averill, 1956). 



(d) The chances of implantation and survival of transferred 

 embryos depends upon a fairly close synchronization between the 

 post-ovulatory age of the uterine environment and the age of the 



