96 FERTILIZATION 



of agents, such as heat, cold and electric shocks, which cause par- 

 thenogenetic activation, induce a release of calcium from the cortex, 

 which in turn starts a cytoplasmic clotting reaction. In confirma- 

 tion of this view, Shaver (1949) found that the foreign agents which 

 must be present as 'needle contaminants' for successful traumatic 

 parthenogenesis in frogs' eggs, favour blood-clotting, while the 

 introduction of the anticoagulant heparin * into the egg inhibits 

 parthenogenesis. Harding (1949) claimed to have confirmed 

 Shaver's results and stated, in addition (1951), that Shear's bacterial 



10 20 30 40 50 



timin.) after fertiLizat'ion 



FIG. 18.^ — Changes in the viscosity of the egg of Chnetopterus variopedatus, at 

 various times after fertilization. The viscosity units are arbitrary, being the 

 number of seconds of centrifugation at 2325 g. required to achieve a particu- 

 lar accumulation of fat granules at the centripetal pole of the egg. A, first 

 polar body; B, late anaphase; C, 2nd maturation prophase; £), 2nd polar 

 body; E, apposition of pronuclei. After Heilbrunn & Wilson (1948). 



polysaccharide, which inhibits blood-clotting, also inhibited par- 

 thenogenesis in the frog's egg. Calcium is a co-factor in that part 

 of the blood-clotting reaction which is concerned with the pro- 

 duction of thrombin and Heilbrunn believes that the release of 

 calcium into the egg cytoplasm promotes the production of a 

 thrombin-like substance. Although there is a little doubt that 

 fertilization and parthenogenetic activation induce changes in the 

 distribution of calcium in the egg and that some form of cortical 

 liquefaction (see also the reference on p. 6 to Allen's work), 

 followed by cytoplasmic gelation, occurs when an egg is activated, 

 the comparative significance of these reactions is not yet clear. 

 Are they, as Heilbrunn believes, the important feature in fertiliza- 



* Heparin is not only an anticoagulant, but is, for example, also concerned 

 with the dissolution of chylomicrons in blood plasma (Florey, 1955). 



