72 THE EGGS OF MAMMALS 



ated by the great acceleration of this dehiscence in the 

 presence of sperm (see Chapter VII). 



The unfertihzed ova of most manamals begin to show- 

 signs of degeneration when they reach the distal portion of 

 the tubes. In the opossum clear evidences of degeneration 

 are observed by twenty-four hours after ovulation when the 



Fig. 20. Fragmenting opossum egg seven 

 days after arriving in the uterus. Section of 

 one of the eggs shown at A, containing three 

 large chromatin masses almost free of cyto- 

 plasm. (From the American Journal of Anat- 

 omy.) 



ova enter the uterus (Hartman, 1924). The degenerative 

 changes have been described in detail by Smith (1925). 

 The ovum may remain intact but develop a well vacuolized 

 cytoplasm with clumped or fragmented chromatin. Ordi- 

 narily, a definite fragmentation of the whole ovum occurs 

 (Figure 20), and the irregular blastomere-like formations 

 may contain bits of fragmented chromatin or lack chromatin 

 entirely. In some 300 opossum ovum sectioned and ex- 

 amined Smith never observed a true cleavage spindle, and 



