STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 85 



by a disorganization and degenerative fragmentation of the egg (see 

 also Thibault, 1949, I95 2 )- Rarely if ever does the nuclear state of 

 such eggs resemble that seen in normal cleavage; the 'blastomercs' 

 contain one or more subnuclei, or apparently no nuclear material 

 at all. Absence of nuclear material from egg fragments suggests that 

 the egg cytoplasm can undergo amitotic division, possibly through 

 the activity of cy tasters. 



Fragmentation of ovarian eggs was found to be more likely to 

 occur in immature animals (Bacsich and Wyburn, 1945), and the 

 frequency increased when the eggs were released from the ovary 

 by artificially-induced ovulation (Austin, 1949b; Chang, 1950c). 

 This might be interpreted as an augmentation of an innate tendency 

 to development, but it seems more reasonable to infer that condi- 

 tions within the immature animal, perhaps more especially within its 

 genital tract, constitute a somewhat unfavourable environment for 

 the egg and conduce to its disorganization. Consistently, it has been 

 found that about one-third of the eggs fertilized in hypophy- 

 sectomized rats (Rowlands and Williams, 1946) and more than half 

 the eggs fertilized in immature rats (Austin, 1950b), after induced 

 ovulation, underwent fragmentation instead of normal cleavage. 

 Degeneration, involving fragmentation, may also be attributable to 

 defects inherent in the eggs (Hartman, 1953). 



Examination of unpenetrated rat eggs reveals that the second 

 meiotic chromosomes become scattered some hours after the normal 

 time of sperm penetration (Fig. 28a), and this occurrence no doubt 

 underlies the subsequent cytoplasmic fragmentation. Delay in the 

 time of fertilization or the application of agents that interfere with 

 the normal organization of chromosomes during cleavage of the 

 fertilized egg may therefore be expected to favour or even promote 

 fragmentation. Increase in the frequency of fragmentation has 

 indeed been found to follow artificial insemination in rats when 

 this is done after the time of ovulation (Odor and Blandau, 1956), 

 and has also been seen as a result of the application of irradiations or 

 radiomimetic agents to spermatozoa before fertilization, although 

 with these treatments the chief effect appeared to be delay of 

 cleavage or even complete arrest of cell division (Brenneke, 1937; 

 Amoroso and Parkes, 1947; Parkes, 1947; Bruce and Austin, 1956; 

 Chang, Hunt and Romanoff, 1958; Edwards, 1957a, b, 1958b). 



