STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 



77 



Fig. 64 

 A 2-cell bat egg showing two 

 nuclei in one blastomere. (Drawn 

 from an illustration by Van der 

 Stricht, 1923.) 



The artificial induction of cleavage of mouse eggs at the second 

 meiosis was reported by Braden and Austin (1954c) who termed 

 the phenomenon 'immediate cleavage'. The effective agent was the 

 application of heat (44 to 45 C C) to the eggs in situ for 5 to 10 min 

 at 8 to 12 hr after ovulation. Five such 

 eggs were seen, representing 7-5 per cent 

 of the eggs recovered. Nine eggs out of 

 a total of 98 recovered from mice sub- 

 jected to deep ether anaesthesia were also 

 judged to have developed through 'im- 

 mediate cleavage', eight of these eggs 

 were 2-cell and one had advanced to 

 the 4-cell stage. When heat treatment 

 was applied to mice 3 to 4 hr after 

 mating, four out of 132 eggs recovered 

 were 2-cell and were considered to have 

 arisen by 'immediate cleavage'; all four 

 contained a spermatozoon and two of 



them had two nuclei in one 'blastomere' and one in the other 

 (Braden and Austin, 1954b). As with cleavage at the first meiosis, 

 the development of mosaic individuals after 'immediate cleavage' is 

 a possibility. Edwards (1958b) has reported twelve instances of 

 penetrated mouse eggs cleaved at the second meiosis, each with 

 two nuclei (pronuclei) in one blastomere and one in the other; the 

 mice had received intrauterine injections of nitrogen mustard just 

 before ovulation and mating. Similar eggs were recovered from 

 mice mated to males that had been injected with triethylenemelamine 

 (Cattanach and Edwards, 1958). 



The penetration of spermatozoa into apparently normal polar 

 bodies has been reported: invertebrates (Wilson, 1928), guinea-pig 

 (Hensen, 1876). Edwards and Sirlin (1959) observed a spermatozoon 

 within a small mass of cytoplasm which resembled a polar body, 

 but they pointed out that in reality the spermatozoon may have 

 entered the vitellus and subsequently been extruded with some of 

 the cytoplasm. The same explanation was put forward by Austin 

 and Braden (1954c) for two rat eggs observed in a similar state. 



In most mammals, the first polar body is emitted shortly before 

 ovulation and the second after the egg has reached the Fallopian 

 tube and as a consequence of sperm penetration, but there are some 

 exceptions to this rule. In the tenrecs (Madagascan insectivores), the 



